6. Manipulating Codes
  
  In  this  chapter  we  describe  several  functions GUAVA uses to manipulate
  codes.  Some  of  the best codes are obtained by starting with for example a
  BCH code, and manipulating it.
  
  In  some cases, it is faster to perform calculations with a manipulated code
  than  to use the original code. For example, if the dimension of the code is
  larger  than  half  the  word  length, it is generally faster to compute the
  weight distribution by first calculating the weight distribution of the dual
  code  than  by  directly calculating the weight distribution of the original
  code. The size of the dual code is smaller in these cases.
  
  Because  GUAVA  keeps  all  information  in a code record, in some cases the
  information can be preserved after manipulations. Therefore, computations do
  not always have to start from scratch.
  
  In  Section  6.1,  we  describe  functions  that  take  a  code with certain
  parameters,  modify  it  in  some  way  and  return  a  different  code (see
  ExtendedCode  (6.1-1),  PuncturedCode  (6.1-2),  EvenWeightSubcode  (6.1-3),
  PermutedCode   (6.1-4),   ExpurgatedCode   (6.1-5),  AugmentedCode  (6.1-6),
  RemovedElementsCode   (6.1-7),   AddedElementsCode   (6.1-8),  ShortenedCode
  (6.1-9),  LengthenedCode  (6.1-10),  ResidueCode (6.1-12), ConstructionBCode
  (6.1-13),      DualCode      (6.1-14),     ConversionFieldCode     (6.1-15),
  ConstantWeightSubcode  (6.1-18),  StandardFormCode  (6.1-19)  and  CosetCode
  (6.1-17)).  In  Section  6.2, we describe functions that generate a new code
  out   of   two   codes   (see   DirectSumCode   (6.2-1),   UUVCode  (6.2-2),
  DirectProductCode (6.2-3), IntersectionCode (6.2-4) and UnionCode (6.2-5)).
  
  
  6.1 Functions that Generate a New Code from a Given Code
  
  6.1-1 ExtendedCode
  
  > ExtendedCode( C[, i] ) ___________________________________________function
  
  ExtendedCode  extends  the code C i times and returns the result. i is equal
  to  1  by  default. Extending is done by adding a parity check bit after the
  last coordinate. The coordinates of all codewords now add up to zero. In the
  binary case, each codeword has even weight.
  
  The  word  length  increases by i. The size of the code remains the same. In
  the  binary  case,  the  minimum distance increases by one if it was odd. In
  other cases, that is not always true.
  
  A cyclic code in general is no longer cyclic after extending.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := HammingCode( 3, GF(2) );
    a linear [7,4,3]1 Hamming (3,2) code over GF(2)
    gap> C2 := ExtendedCode( C1 );
    a linear [8,4,4]2 extended code
    gap> IsEquivalent( C2, ReedMullerCode( 1, 3 ) );
    true
    gap> List( AsSSortedList( C2 ), WeightCodeword );
    [ 0, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 8 ]
    gap> C3 := EvenWeightSubcode( C1 );
    a linear [7,3,4]2..3 even weight subcode 
  ------------------------------------------------------------------
  
  To  undo  extending,  call  PuncturedCode  (see  PuncturedCode (6.1-2)). The
  function  EvenWeightSubcode  (see  EvenWeightSubcode (6.1-3)) also returns a
  related code with only even weights, but without changing its word length.
  
  6.1-2 PuncturedCode
  
  > PuncturedCode( C ) _______________________________________________function
  
  PuncturedCode  punctures  C  in  the  last  column,  and returns the result.
  Puncturing is done simply by cutting off the last column from each codeword.
  This means the word length decreases by one. The minimum distance in general
  also decrease by one.
  
  This  command  can  also be called with the syntax PuncturedCode( C, L ). In
  this  case,  PuncturedCode punctures C in the columns specified by L, a list
  of integers. All columns specified by L are omitted from each codeword. If l
  is  the  length  of  L  (so  the number of removed columns), the word length
  decreases by l. The minimum distance can also decrease by l or less.
  
  Puncturing  a  cyclic  code  in general results in a non-cyclic code. If the
  code  is  punctured  in  all  the  columns where a word of minimal weight is
  unequal to zero, the dimension of the resulting code decreases.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := BCHCode( 15, 5, GF(2) );
    a cyclic [15,7,5]3..5 BCH code, delta=5, b=1 over GF(2)
    gap> C2 := PuncturedCode( C1 );
    a linear [14,7,4]3..5 punctured code
    gap> ExtendedCode( C2 ) = C1;
    false
    gap> PuncturedCode( C1, [1,2,3,4,5,6,7] );
    a linear [8,7,1]1 punctured code
    gap> PuncturedCode( WholeSpaceCode( 4, GF(5) ) );
    a linear [3,3,1]0 punctured code  # The dimension decreased from 4 to 3 
  ------------------------------------------------------------------
  
  ExtendedCode  extends the code again (see ExtendedCode (6.1-1)), although in
  general this does not result in the old code.
  
  6.1-3 EvenWeightSubcode
  
  > EvenWeightSubcode( C ) ___________________________________________function
  
  EvenWeightSubcode  returns  the  even weight subcode of C, consisting of all
  codewords of C with even weight. If C is a linear code and contains words of
  odd  weight,  the  resulting  code  has a dimension of one less. The minimum
  distance  always  increases  with one if it was odd. If C is a binary cyclic
  code,  and  g(x) is its generator polynomial, the even weight subcode either
  has  generator  polynomial g(x) (if g(x) is divisible by x-1) or g(x)* (x-1)
  (if  no factor x-1 was present in g(x)). So the even weight subcode is again
  cyclic.
  
  Of  course,  if  all codewords of C are already of even weight, the returned
  code is equal to C.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := EvenWeightSubcode( BCHCode( 8, 4, GF(3) ) );
    an (8,33,4..8)3..8 even weight subcode
    gap> List( AsSSortedList( C1 ), WeightCodeword );
    [ 0, 4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 6, 4, 4, 6, 4, 4, 8, 6, 4, 6, 8, 4, 4, 
      4, 6, 4, 6, 8, 4, 6, 8 ]
    gap> EvenWeightSubcode( ReedMullerCode( 1, 3 ) );
    a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2) 
  ------------------------------------------------------------------
  
  ExtendedCode  also  returns a related code of only even weights, but without
  reducing its dimension (see ExtendedCode (6.1-1)).
  
  6.1-4 PermutedCode
  
  > PermutedCode( C, L ) _____________________________________________function
  
  PermutedCode  returns  C after column permutations. L (in GAP disjoint cycle
  notation)  is  the  permutation  to be executed on the columns of C. If C is
  cyclic,  the result in general is no longer cyclic. If a permutation results
  in the same code as C, this permutation belongs to the automorphism group of
  C  (see  AutomorphismGroup  (4.4-3)).  In  any  case,  the  returned code is
  equivalent to C (see IsEquivalent (4.4-1)).
  
  ---------------------------  Example  ----------------------------
    gap> C1 := PuncturedCode( ReedMullerCode( 1, 4 ) );
    a linear [15,5,7]5 punctured code
    gap> C2 := BCHCode( 15, 7, GF(2) );
    a cyclic [15,5,7]5 BCH code, delta=7, b=1 over GF(2)
    gap> C2 = C1;
    false
    gap> p := CodeIsomorphism( C1, C2 );
    ( 2, 4,14, 9,13, 7,11,10, 6, 8,12, 5)
    gap> C3 := PermutedCode( C1, p );
    a linear [15,5,7]5 permuted code
    gap> C2 = C3;
    true 
  ------------------------------------------------------------------
  
  6.1-5 ExpurgatedCode
  
  > ExpurgatedCode( C, L ) ___________________________________________function
  
  ExpurgatedCode  expurgates the code C> by throwing away codewords in list L.
  C  must  be a linear code. L must be a list of codeword input. The generator
  matrix  of  the new code no longer is a basis for the codewords specified by
  L.  Since the returned code is still linear, it is very likely that, besides
  the words of L, more codewords of C are no longer in the new code.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := HammingCode( 4 );; WeightDistribution( C1 );
    [ 1, 0, 0, 35, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
    gap> L := Filtered( AsSSortedList(C1), i -> WeightCodeword(i) = 3 );;
    gap> C2 := ExpurgatedCode( C1, L );
    a linear [15,4,3..4]5..11 code, expurgated with 7 word(s)
    gap> WeightDistribution( C2 );
    [ 1, 0, 0, 0, 14, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0 ] 
  ------------------------------------------------------------------
  
  This  function  does not work on non-linear codes. For removing words from a
  non-linear  code, use RemovedElementsCode (see RemovedElementsCode (6.1-7)).
  For  expurgating  a code of all words of odd weight, use `EvenWeightSubcode'
  (see EvenWeightSubcode (6.1-3)).
  
  6.1-6 AugmentedCode
  
  > AugmentedCode( C, L ) ____________________________________________function
  
  AugmentedCode returns C after augmenting. C must be a linear code, L must be
  a  list  of codeword inputs. The generator matrix of the new code is a basis
  for  the  codewords specified by L as well as the words that were already in
  code  C.  Note  that the new code in general will consist of more words than
  only  the codewords of C and the words L. The returned code is also a linear
  code.
  
  This  command  can  also  be  called  with the syntax AugmentedCode(C). When
  called without a list of codewords, AugmentedCode returns C after adding the
  all-ones  vector  to  the  generator matrix. C must be a linear code. If the
  all-ones  vector  was already in the code, nothing happens and a copy of the
  argument  is  returned.  If  C  is  a binary code which does not contain the
  all-ones vector, the complement of all codewords is added.
  
  ---------------------------  Example  ----------------------------
    gap> C31 := ReedMullerCode( 1, 3 );
    a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
    gap> C32 := AugmentedCode(C31,["00000011","00000101","00010001"]);
    a linear [8,7,1..2]1 code, augmented with 3 word(s)
    gap> C32 = ReedMullerCode( 2, 3 );
    true 
    gap> C1 := CordaroWagnerCode(6);
    a linear [6,2,4]2..3 Cordaro-Wagner code over GF(2)
    gap> Codeword( [0,0,1,1,1,1] ) in C1;
    true
    gap> C2 := AugmentedCode( C1 );
    a linear [6,3,1..2]2..3 code, augmented with 1 word(s)
    gap> Codeword( [1,1,0,0,0,0] ) in C2;
    true
  ------------------------------------------------------------------
  
  The  function  AddedElementsCode  adds  elements to the codewords instead of
  adding them to the basis (see AddedElementsCode (6.1-8)).
  
  6.1-7 RemovedElementsCode
  
  > RemovedElementsCode( C, L ) ______________________________________function
  
  RemovedElementsCode returns code C after removing a list of codewords L from
  its  elements.  L  must  be  a  list  of  codeword  input.  The result is an
  unrestricted code.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := HammingCode( 4 );; WeightDistribution( C1 );
    [ 1, 0, 0, 35, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
    gap> L := Filtered( AsSSortedList(C1), i -> WeightCodeword(i) = 3 );;
    gap> C2 := RemovedElementsCode( C1, L );
    a (15,2013,3..15)2..15 code with 35 word(s) removed
    gap> WeightDistribution( C2 );
    [ 1, 0, 0, 0, 105, 168, 280, 435, 435, 280, 168, 105, 35, 0, 0, 1 ]
    gap> MinimumDistance( C2 );
    3        # C2 is not linear, so the minimum weight does not have to
             # be equal to the minimum distance 
  ------------------------------------------------------------------
  
  Adding  elements  to  a  code is done by the function AddedElementsCode (see
  AddedElementsCode  (6.1-8)).  To  remove codewords from the base of a linear
  code, use ExpurgatedCode (see ExpurgatedCode (6.1-5)).
  
  6.1-8 AddedElementsCode
  
  > AddedElementsCode( C, L ) ________________________________________function
  
  AddedElementsCode  returns  code C after adding a list of codewords L to its
  elements.  L must be a list of codeword input. The result is an unrestricted
  code.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := NullCode( 6, GF(2) );
    a cyclic [6,0,6]6 nullcode over GF(2)
    gap> C2 := AddedElementsCode( C1, [ "111111" ] );
    a (6,2,1..6)3 code with 1 word(s) added
    gap> IsCyclicCode( C2 );
    true
    gap> C3 := AddedElementsCode( C2, [ "101010", "010101" ] );
    a (6,4,1..6)2 code with 2 word(s) added
    gap> IsCyclicCode( C3 );
    true 
  ------------------------------------------------------------------
  
  To   remove   elements   from   a   code,   use   RemovedElementsCode   (see
  RemovedElementsCode  (6.1-7)). To add elements to the base of a linear code,
  use AugmentedCode (see AugmentedCode (6.1-6)).
  
  6.1-9 ShortenedCode
  
  > ShortenedCode( C[, L] ) __________________________________________function
  
  ShortenedCode(  C  ) returns the code C shortened by taking a cross section.
  If  C  is  a  linear code, this is done by removing all codewords that start
  with  a  non-zero entry, after which the first column is cut off. If C was a
  [n,k,d]  code,  the  shortened  code  generally is a [n-1,k-1,d] code. It is
  possible  that  the dimension remains the same; it is also possible that the
  minimum distance increases.
  
  If  C  is  a  non-linear code, ShortenedCode first checks which finite field
  element  occurs  most  often  in  the  first  column  of  the codewords. The
  codewords  not  starting  with this element are removed from the code, after
  which the first column is cut off. The resulting shortened code has at least
  the same minimum distance as C.
  
  This  command  can  also be called using the syntax ShortenedCode(C,L). When
  called  in this format, ShortenedCode repeats the shortening process on each
  of the columns specified by L. L therefore is a list of integers. The column
  numbers in L are the numbers as they are before the shortening process. If L
  has  l  entries,  the returned code has a word length of l positions shorter
  than C.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := HammingCode( 4 );
    a linear [15,11,3]1 Hamming (4,2) code over GF(2)
    gap> C2 := ShortenedCode( C1 );
    a linear [14,10,3]2 shortened code
    gap> C3 := ElementsCode( ["1000", "1101", "0011" ], GF(2) );
    a (4,3,1..4)2 user defined unrestricted code over GF(2)
    gap> MinimumDistance( C3 );
    2
    gap> C4 := ShortenedCode( C3 );
    a (3,2,2..3)1..2 shortened code
    gap> AsSSortedList( C4 );
    [ [ 0 0 0 ], [ 1 0 1 ] ]
    gap> C5 := HammingCode( 5, GF(2) );
    a linear [31,26,3]1 Hamming (5,2) code over GF(2)
    gap> C6 := ShortenedCode( C5, [ 1, 2, 3 ] );
    a linear [28,23,3]2 shortened code
    gap> OptimalityLinearCode( C6 );
    0
  ------------------------------------------------------------------
  
  The  function  LengthenedCode  lengthens  the  code  again  (only for linear
  codes),  see  LengthenedCode  (6.1-10).  In general, this is not exactly the
  inverse function.
  
  6.1-10 LengthenedCode
  
  > LengthenedCode( C[, i] ) _________________________________________function
  
  LengthenedCode(  C ) returns the code C lengthened. C must be a linear code.
  First,   the   all-ones  vector  is  added  to  the  generator  matrix  (see
  AugmentedCode  (6.1-6)).  If  the  all-ones  vector  was already a codeword,
  nothing  happens  to  the  code.  Then,  the  code  is extended i times (see
  ExtendedCode  (6.1-1)).  i is equal to 1 by default. If C was an [n,k] code,
  the new code generally is a [n+i,k+1] code.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := CordaroWagnerCode( 5 );
    a linear [5,2,3]2 Cordaro-Wagner code over GF(2)
    gap> C2 := LengthenedCode( C1 );
    a linear [6,3,2]2..3 code, lengthened with 1 column(s) 
  ------------------------------------------------------------------
  
  ShortenedCode'  shortens  the  code,  see ShortenedCode (6.1-9). In general,
  this is not exactly the inverse function.
  
  6.1-11 SubCode
  
  > SubCode( C[, s] ) ________________________________________________function
  
  This  function SubCode returns a subcode of C by taking the first k - s rows
  of  the generator matrix of C, where k is the dimension of C. The interger s
  may be omitted and in this case it is assumed as 1.
  
  ---------------------------  Example  ----------------------------
    gap> C := BCHCode(31,11);
    a cyclic [31,11,11]7..11 BCH code, delta=11, b=1 over GF(2)
    gap> S1:= SubCode(C);
    a linear [31,10,11]7..13 subcode
    gap> WeightDistribution(S1);
    [ 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 120, 190, 0, 0, 272, 255, 0, 0, 120, 66,
      0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ]
    gap> S2:= SubCode(C, 8);
    a linear [31,3,11]14..20 subcode
    gap> History(S2);
    [ "a linear [31,3,11]14..20 subcode of",
      "a cyclic [31,11,11]7..11 BCH code, delta=11, b=1 over GF(2)" ]
    gap> WeightDistribution(S2);
    [ 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0,
      0, 0, 0, 0, 0, 0, 0 ]
  ------------------------------------------------------------------
  
  6.1-12 ResidueCode
  
  > ResidueCode( C[, c] ) ____________________________________________function
  
  The  function  ResidueCode  takes  a  codeword  c  of  C (if c is omitted, a
  codeword of minimal weight is used). It removes this word and all its linear
  combinations  from  the  code and then punctures the code in the coordinates
  where  c  is  unequal  to zero. The resulting code is an [n-w, k-1, d-lfloor
  w*(q-1)/q  rfloor ] code. C must be a linear code and c must be non-zero. If
  c is not in  then no change is made to C.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := BCHCode( 15, 7 );
    a cyclic [15,5,7]5 BCH code, delta=7, b=1 over GF(2)
    gap> C2 := ResidueCode( C1 );
    a linear [8,4,4]2 residue code
    gap> c := Codeword( [ 0,0,0,1,0,0,1,1,0,1,0,1,1,1,1 ], C1);;
    gap> C3 := ResidueCode( C1, c );
    a linear [7,4,3]1 residue code 
  ------------------------------------------------------------------
  
  6.1-13 ConstructionBCode
  
  > ConstructionBCode( C ) ___________________________________________function
  
  The  function  ConstructionBCode takes a binary linear code C and calculates
  the  minimum  distance  of  the  dual  of C (see DualCode (6.1-14)). It then
  removes  the columns of the parity check matrix of C where a codeword of the
  dual  code  of minimal weight has coordinates unequal to zero. The resulting
  matrix  is  a parity check matrix for an [n-dd, k-dd+1, >= d] code, where dd
  is the minimum distance of the dual of C.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := ReedMullerCode( 2, 5 );
    a linear [32,16,8]6 Reed-Muller (2,5) code over GF(2)
    gap> C2 := ConstructionBCode( C1 );
    a linear [24,9,8]5..10 Construction B (8 coordinates)
    gap> BoundsMinimumDistance( 24, 9, GF(2) );
    rec( n := 24, k := 9, q := 2, references := rec(  ), 
      construction := [ [ Operation "UUVCode" ], 
          [ [ [ Operation "UUVCode" ], [ [ [ Operation "DualCode" ], 
                          [ [ [ Operation "RepetitionCode" ], [ 6, 2 ] ] ] ], 
                      [ [ Operation "CordaroWagnerCode" ], [ 6 ] ] ] ], 
              [ [ Operation "CordaroWagnerCode" ], [ 12 ] ] ] ], lowerBound := 8, 
      lowerBoundExplanation := [ "Lb(24,9)=8, u u+v construction of C1 and C2:", 
          "Lb(12,7)=4, u u+v construction of C1 and C2:", 
          "Lb(6,5)=2, dual of the repetition code", 
          "Lb(6,2)=4, Cordaro-Wagner code", "Lb(12,2)=8, Cordaro-Wagner code" ], 
      upperBound := 8, 
      upperBoundExplanation := [ "Ub(24,9)=8, otherwise construction B would 
                                 contradict:", "Ub(18,4)=8, Griesmer bound" ] )
    # so C2 is optimal
  ------------------------------------------------------------------
  
  6.1-14 DualCode
  
  > DualCode( C ) ____________________________________________________function
  
  DualCode returns the dual code of C. The dual code consists of all codewords
  that  are  orthogonal  to  the  codewords  of  C. If C is a linear code with
  generator  matrix  G,  the  dual code has parity check matrix G (or if C has
  parity  check  matrix H, the dual code has generator matrix H). So if C is a
  linear  [n, k] code, the dual code of C is a linear [n, n-k] code. If C is a
  cyclic code with generator polynomial g(x), the dual code has the reciprocal
  polynomial of g(x) as check polynomial.
  
  The dual code is always a linear code, even if C is non-linear.
  
  If a code C is equal to its dual code, it is called self-dual.
  
  ---------------------------  Example  ----------------------------
    gap> R := ReedMullerCode( 1, 3 );
    a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
    gap> RD := DualCode( R );
    a linear [8,4,4]2 Reed-Muller (1,3) code over GF(2)
    gap> R = RD;
    true
    gap> N := WholeSpaceCode( 7, GF(4) );
    a cyclic [7,7,1]0 whole space code over GF(4)
    gap> DualCode( N ) = NullCode( 7, GF(4) );
    true 
  ------------------------------------------------------------------
  
  6.1-15 ConversionFieldCode
  
  > ConversionFieldCode( C ) _________________________________________function
  
  ConversionFieldCode  returns  the  code obtained from C after converting its
  field. If the field of C is GF(q^m), the returned code has field GF(q). Each
  symbol  of  every  codeword is replaced by a concatenation of m symbols from
  GF(q).  If  C  is an (n, M, d_1) code, the returned code is a (n* m, M, d_2)
  code, where d_2 > d_1.
  
  See also HorizontalConversionFieldMat (7.3-10).
  
  ---------------------------  Example  ----------------------------
    gap> R := RepetitionCode( 4, GF(4) );
    a cyclic [4,1,4]3 repetition code over GF(4)
    gap> R2 := ConversionFieldCode( R );
    a linear [8,2,4]3..4 code, converted to basefield GF(2)
    gap> Size( R ) = Size( R2 );
    true
    gap> GeneratorMat( R );
    [ [ Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0 ] ]
    gap> GeneratorMat( R2 );
    [ [ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2) ],
      [ 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0 ] ] 
  ------------------------------------------------------------------
  
  6.1-16 TraceCode
  
  > TraceCode( C ) ___________________________________________________function
  
  Input:  C is a linear code defined over an extension E of F (F is the ``base
  field'')
  
  Output: The linear code generated by Tr_E/F(c), for all c in C.
  
  TraceCode  returns the image of the code C under the trace map. If the field
  of C is GF(q^m), the returned code has field GF(q).
  
  Very  slow.  It  does  not  seem to be easy to related the parameters of the
  trace code to the original except in the ``Galois closed'' case.
  
  ---------------------------  Example  ----------------------------
    gap> C:=RandomLinearCode(10,4,GF(4)); MinimumDistance(C);
    a  [10,4,?] randomly generated code over GF(4)
    5
    gap> trC:=TraceCode(C,GF(2)); MinimumDistance(trC);
    a linear [10,7,1]1..3 user defined unrestricted code over GF(2)
    1
    
  ------------------------------------------------------------------
  
  6.1-17 CosetCode
  
  > CosetCode( C, w ) ________________________________________________function
  
  CosetCode returns the coset of a code C with respect to word w. w must be of
  the  codeword  type.  Then,  w  is added to each codeword of C, yielding the
  elements  of  the new code. If C is linear and w is an element of C, the new
  code is equal to C, otherwise the new code is an unrestricted code.
  
  Generating  a  coset  is also possible by simply adding the word w to C. See
  4.2.
  
  ---------------------------  Example  ----------------------------
    gap> H := HammingCode(3, GF(2));
    a linear [7,4,3]1 Hamming (3,2) code over GF(2)
    gap> c := Codeword("1011011");; c in H;
    false
    gap> C := CosetCode(H, c);
    a (7,16,3)1 coset code
    gap> List(AsSSortedList(C), el-> Syndrome(H, el));
    [ [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ],
      [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ],
      [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ], [ 1 1 1 ] ]
    # All elements of the coset have the same syndrome in H 
  ------------------------------------------------------------------
  
  6.1-18 ConstantWeightSubcode
  
  > ConstantWeightSubcode( C, w ) ____________________________________function
  
  ConstantWeightSubcode  returns  the  subcode of C that only has codewords of
  weight  w.  The  resulting  code  is  a non-linear code, because it does not
  contain the all-zero vector.
  
  This  command also can be called with the syntax ConstantWeightSubcode(C) In
  this  format,  ConstantWeightSubcode  returns the subcode of C consisting of
  all minimum weight codewords of C.
  
  ConstantWeightSubcode  first  checks  if Leon's binary wtdist exists on your
  computer  (in  the  default  directory).  If  it  does, then this program is
  called.  Otherwise,  the  constant  weight  subcode  is computed using a GAP
  program  which  checks  each  codeword  in  C to see if it is of the desired
  weight.
  
  ---------------------------  Example  ----------------------------
    gap> N := NordstromRobinsonCode();; WeightDistribution(N);
    [ 1, 0, 0, 0, 0, 0, 112, 0, 30, 0, 112, 0, 0, 0, 0, 0, 1 ]
    gap> C := ConstantWeightSubcode(N, 8);
    a (16,30,6..16)5..8 code with codewords of weight 8
    gap> WeightDistribution(C);
    [ 0, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0, 0, 0, 0 ] 
    gap> eg := ExtendedTernaryGolayCode();; WeightDistribution(eg);
    [ 1, 0, 0, 0, 0, 0, 264, 0, 0, 440, 0, 0, 24 ]
    gap> C := ConstantWeightSubcode(eg);
    a (12,264,6..12)3..6 code with codewords of weight 6
    gap> WeightDistribution(C);
    [ 0, 0, 0, 0, 0, 0, 264, 0, 0, 0, 0, 0, 0 ] 
  ------------------------------------------------------------------
  
  6.1-19 StandardFormCode
  
  > StandardFormCode( C ) ____________________________________________function
  
  StandardFormCode  returns  C  after  putting  it in standard form. If C is a
  non-linear code, this means the elements are organized using lexicographical
  order. This means they form a legal GAP `Set'.
  
  If  C is a linear code, the generator matrix and parity check matrix are put
  in  standard  form.  The generator matrix then has an identity matrix in its
  left part, the parity check matrix has an identity matrix in its right part.
  Although  GUAVA  always puts both matrices in a standard form using BaseMat,
  this   never   alters   the   code.  StandardFormCode  even  applies  column
  permutations  if  unavoidable,  and  thereby  changes  the  code. The column
  permutations  are  recorded in the construction history of the new code (see
  Display (4.6-3)). C and the new code are of course equivalent.
  
  If C is a cyclic code, its generator matrix cannot be put in the usual upper
  triangular  form,  because  then it would be inconsistent with the generator
  polynomial.  The  reason  is that generating the elements from the generator
  matrix  would  result in a different order than generating the elements from
  the  generator  polynomial.  This  is  an  unwanted  effect,  and  therefore
  StandardFormCode just returns a copy of C for cyclic codes.
  
  ---------------------------  Example  ----------------------------
    gap> G := GeneratorMatCode( Z(2) * [ [0,1,1,0], [0,1,0,1], [0,0,1,1] ], 
              "random form code", GF(2) );
    a linear [4,2,1..2]1..2 random form code over GF(2)
    gap> Codeword( GeneratorMat( G ) );
    [ [ 0 1 0 1 ], [ 0 0 1 1 ] ]
    gap> Codeword( GeneratorMat( StandardFormCode( G ) ) );
    [ [ 1 0 0 1 ], [ 0 1 0 1 ] ] 
  ------------------------------------------------------------------
  
  6.1-20 PiecewiseConstantCode
  
  > PiecewiseConstantCode( part, wts[, F] ) __________________________function
  
  PiecewiseConstantCode  returns  a code with length n = sum n_i, where part=[
  n_1,  dots,  n_k  ].  wts  is a list of constraints w=(w_1,...,w_k), each of
  length k, where 0 <= w_i <= n_i. The default field is GF(2).
  
  A  constraint  is  a  list  of  integers,  and a word c = ( c_1, dots, c_k )
  (according  to  part,  i.e.,  each c_i is a subword of length n_i) is in the
  resulting code if and only if, for some constraint w in wts, |c_i| = w_i for
  all 1 <= i <= k, where | ...| denotes the Hamming weight.
  
  An example might make things clearer:
  
  ---------------------------  Example  ----------------------------
    gap> PiecewiseConstantCode( [ 2, 3 ],
         [ [ 0, 0 ], [ 0, 3 ], [ 1, 0 ], [ 2, 2 ] ],GF(2) );
    the C code programs are compiled, so using Leon's binary....
    the C code programs are compiled, so using Leon's binary....
    the C code programs are compiled, so using Leon's binary....
    the C code programs are compiled, so using Leon's binary....
    a (5,7,1..5)1..5 piecewise constant code over GF(2)
    gap> AsSSortedList(last);
    [ [ 0 0 0 0 0 ], [ 0 0 1 1 1 ], [ 0 1 0 0 0 ], [ 1 0 0 0 0 ], 
      [ 1 1 0 1 1 ], [ 1 1 1 0 1 ], [ 1 1 1 1 0 ] ]
    gap>
    
  ------------------------------------------------------------------
  
  The  first  constraint is satisfied by codeword 1, the second by codeword 2,
  the third by codewords 3 and 4, and the fourth by codewords 5, 6 and 7.
  
  
  6.2 Functions that Generate a New Code from Two or More Given Codes
  
  6.2-1 DirectSumCode
  
  > DirectSumCode( C1, C2 ) __________________________________________function
  
  DirectSumCode returns the direct sum of codes C1 and C2. The direct sum code
  consists  of  every  codeword  of  C1  concatenated by every codeword of C2.
  Therefore,   if   Ci   was   a   (n_i,M_i,d_i)   code,   the   result  is  a
  (n_1+n_2,M_1*M_2,min(d_1,d_2)) code.
  
  If both C1 and C2 are linear codes, the result is also a linear code. If one
  of  them  is  non-linear,  the  direct  sum is non-linear too. In general, a
  direct sum code is not cyclic.
  
  Performing  a  direct  sum can also be done by adding two codes (see Section
  4.2).  Another  often used method is the `u, u+v'-construction, described in
  UUVCode (6.2-2).
  
  ---------------------------  Example  ----------------------------
    gap> C1 := ElementsCode( [ [1,0], [4,5] ], GF(7) );;
    gap> C2 := ElementsCode( [ [0,0,0], [3,3,3] ], GF(7) );;
    gap> D := DirectSumCode(C1, C2);;
    gap> AsSSortedList(D);
    [ [ 1 0 0 0 0 ], [ 1 0 3 3 3 ], [ 4 5 0 0 0 ], [ 4 5 3 3 3 ] ]
    gap> D = C1 + C2;   # addition = direct sum
    true 
  ------------------------------------------------------------------
  
  6.2-2 UUVCode
  
  > UUVCode( C1, C2 ) ________________________________________________function
  
  UUVCode returns the so-called (u|u+v) construction applied to C1 and C2. The
  resulting code consists of every codeword u of C1 concatenated by the sum of
  u  and  every  codeword  v  of C2. If C1 and C2 have different word lengths,
  sufficient zeros are added to the shorter code to make this sum possible. If
  Ci  is  a  (n_i,M_i,d_i)  code,  the  result  is  an  (n_1+max(n_1,n_2),M_1*
  M_2,min(2* d_1,d_2)) code.
  
  If both C1 and C2 are linear codes, the result is also a linear code. If one
  of  them is non-linear, the UUV sum is non-linear too. In general, a UUV sum
  code is not cyclic.
  
  The  function  DirectSumCode returns another sum of codes (see DirectSumCode
  (6.2-1)).
  
  ---------------------------  Example  ----------------------------
    gap> C1 := EvenWeightSubcode(WholeSpaceCode(4, GF(2)));
    a cyclic [4,3,2]1 even weight subcode
    gap> C2 := RepetitionCode(4, GF(2));
    a cyclic [4,1,4]2 repetition code over GF(2)
    gap> R := UUVCode(C1, C2);
    a linear [8,4,4]2 U U+V construction code
    gap> R = ReedMullerCode(1,3);
    true 
  ------------------------------------------------------------------
  
  6.2-3 DirectProductCode
  
  > DirectProductCode( C1, C2 ) ______________________________________function
  
  DirectProductCode  returns  the direct product of codes C1 and C2. Both must
  be  linear codes. Suppose Ci has generator matrix G_i. The direct product of
  C1  and  C2  then  has the Kronecker product of G_1 and G_2 as the generator
  matrix (see the GAP command KroneckerProduct).
  
  If  Ci  is  a  [n_i,  k_i,  d_i]  code,  the direct product then is an [n_1*
  n_2,k_1* k_2,d_1* d_2] code.
  
  ---------------------------  Example  ----------------------------
    gap> L1 := LexiCode(10, 4, GF(2));
    a linear [10,5,4]2..4 lexicode over GF(2)
    gap> L2 := LexiCode(8, 3, GF(2));
    a linear [8,4,3]2..3 lexicode over GF(2)
    gap> D := DirectProductCode(L1, L2);
    a linear [80,20,12]20..45 direct product code 
  ------------------------------------------------------------------
  
  6.2-4 IntersectionCode
  
  > IntersectionCode( C1, C2 ) _______________________________________function
  
  IntersectionCode  returns  the  intersection  of  codes C1 and C2. This code
  consists  of  all  codewords  that  are both in C1 and C2. If both codes are
  linear,  the  result  is also linear. If both are cyclic, the result is also
  cyclic.
  
  ---------------------------  Example  ----------------------------
    gap> C := CyclicCodes(7, GF(2));
    [ a cyclic [7,7,1]0 enumerated code over GF(2),
      a cyclic [7,6,1..2]1 enumerated code over GF(2),
      a cyclic [7,3,1..4]2..3 enumerated code over GF(2),
      a cyclic [7,0,7]7 enumerated code over GF(2),
      a cyclic [7,3,1..4]2..3 enumerated code over GF(2),
      a cyclic [7,4,1..3]1 enumerated code over GF(2),
      a cyclic [7,1,7]3 enumerated code over GF(2),
      a cyclic [7,4,1..3]1 enumerated code over GF(2) ]
    gap> IntersectionCode(C[6], C[8]) = C[7];
    true 
  ------------------------------------------------------------------
  
  The  hull  of  a  linear  code is the intersection of the code with its dual
  code. In other words, the hull of C is IntersectionCode(C, DualCode(C)).
  
  6.2-5 UnionCode
  
  > UnionCode( C1, C2 ) ______________________________________________function
  
  UnionCode  returns  the  union of codes C1 and C2. This code consists of the
  union  of  all codewords of C1 and C2 and all linear combinations. Therefore
  this  function  works  only for linear codes. The function AddedElementsCode
  can  be  used  for  non-linear  codes,  or  if the resulting code should not
  include   linear   combinations.  See  AddedElementsCode  (6.1-8).  If  both
  arguments are cyclic, the result is also cyclic.
  
  ---------------------------  Example  ----------------------------
    gap> G := GeneratorMatCode([[1,0,1],[0,1,1]]*Z(2)^0, GF(2));
    a linear [3,2,1..2]1 code defined by generator matrix over GF(2)
    gap> H := GeneratorMatCode([[1,1,1]]*Z(2)^0, GF(2));
    a linear [3,1,3]1 code defined by generator matrix over GF(2)
    gap> U := UnionCode(G, H);
    a linear [3,3,1]0 union code
    gap> c := Codeword("010");; c in G;
    false
    gap> c in H;
    false
    gap> c in U;
    true 
  ------------------------------------------------------------------
  
  6.2-6 ExtendedDirectSumCode
  
  > ExtendedDirectSumCode( L, B, m ) _________________________________function
  
  The extended direct sum construction is described in section V of Graham and
  Sloane  [GS85].  The  resulting  code consists of m copies of L, extended by
  repeating the codewords of B m times.
  
  Suppose  L  is  an  [n_L,  k_L]r_L  code,  and  B  is  an [n_L, k_B]r_B code
  (non-linear  codes are also permitted). The length of B must be equal to the
  length  of L. The length of the new code is n = m n_L, the dimension (in the
  case  of  linear codes) is k <= m k_L + k_B, and the covering radius is r <=
  lfloor m Psi( L, B ) rfloor, with
  
  
       \Psi( L, B ) = \max_{u \in F_2^{n_L}} \frac{1}{2^{k_B}} \sum_{v
       \in B} {\rm d}( L, v + u ).
  
  
  However,  this  computation will not be executed, because it may be too time
  consuming for large codes.
  
  If  L  subseteq  B,  and  L  and  B  are linear codes, the last copy of L is
  omitted. In this case the dimension is k = m k_L + (k_B - k_L).
  
  ---------------------------  Example  ----------------------------
    gap> c := HammingCode( 3, GF(2) );
    a linear [7,4,3]1 Hamming (3,2) code over GF(2)
    gap> d := WholeSpaceCode( 7, GF(2) );
    a cyclic [7,7,1]0 whole space code over GF(2)
    gap> e := ExtendedDirectSumCode( c, d, 3 );
    a linear [21,15,1..3]2 3-fold extended direct sum code
  ------------------------------------------------------------------
  
  6.2-7 AmalgamatedDirectSumCode
  
  > AmalgamatedDirectSumCode( c1, c2[, check] ) ______________________function
  
  AmalgamatedDirectSumCode  returns the amalgamated direct sum of the codes c1
  and  c2.  The  amalgamated  direct sum code consists of all codewords of the
  form (u , | ,0 , | , v) if (u , | , 0) in c_1 and (0 , | , v) in c_2 and all
  codewords of the form (u , | , 1 , | , v) if (u , | , 1) in c_1 and (1 , | ,
  v)  in c_2. The result is a code with length n = n_1 + n_2 - 1 and size M <=
  M_1 * M_2 / 2.
  
  If  both codes are linear, they will first be standardized, with information
  symbols  in  the  last  and  first coordinates of the first and second code,
  respectively.
  
  If  c1  is a normal code (see IsNormalCode (7.4-5)) with the last coordinate
  acceptable  (see  IsCoordinateAcceptable  (7.4-3)),  and c2 is a normal code
  with  the  first  coordinate acceptable, then the covering radius of the new
  code is r <= r_1 + r_2. However, checking whether a code is normal or not is
  a  lot of work, and almost all codes seem to be normal. Therefore, an option
  check  can be supplied. If check is true, then the codes will be checked for
  normality. If check is false or omitted, then the codes will not be checked.
  In  this  case it is assumed that they are normal. Acceptability of the last
  and  first  coordinate of the first and second code, respectively, is in the
  last case also assumed to be done by the user.
  
  ---------------------------  Example  ----------------------------
    gap> c := HammingCode( 3, GF(2) );
    a linear [7,4,3]1 Hamming (3,2) code over GF(2)
    gap> d := ReedMullerCode( 1, 4 );
    a linear [16,5,8]6 Reed-Muller (1,4) code over GF(2)
    gap> e := DirectSumCode( c, d );
    a linear [23,9,3]7 direct sum code
    gap> f := AmalgamatedDirectSumCode( c, d );;
    gap> MinimumDistance( f );;
    gap> CoveringRadius( f );; 
    gap> f;
    a linear [22,8,3]7 amalgamated direct sum code
  ------------------------------------------------------------------
  
  6.2-8 BlockwiseDirectSumCode
  
  > BlockwiseDirectSumCode( C1, L1, C2, L2 ) _________________________function
  
  BlockwiseDirectSumCode returns a subcode of the direct sum of C1 and C2. The
  fields  of  C1 and C2 must be same. The lists L1 and L2 are two equally long
  with  elements from the ambient vector spaces of C1 and C2, respectively, or
  L1  and  L2  are  two  equally long lists containing codes. The union of the
  codes in L1 and L2 must be C1 and C2, respectively.
  
  In the first case, the blockwise direct sum code is defined as
  
  
       bds = \bigcup_{1 \leq i \leq \ell} ( C_1 + (L_1)_i ) \oplus ( C_2
       + (L_2)_i ),
  
  
  where ell is the length of L1 and L2, and oplus is the direct sum.
  
  In the second case, it is defined as
  
  
       bds = \bigcup_{1 \leq i \leq \ell} ( (L_1)_i \oplus (L_2)_i ).
  
  
  The length of the new code is n = n_1 + n_2.
  
  ---------------------------  Example  ----------------------------
    gap> C1 := HammingCode( 3, GF(2) );;
    gap> C2 := EvenWeightSubcode( WholeSpaceCode( 6, GF(2) ) );;
    gap> BlockwiseDirectSumCode( C1, [[ 0,0,0,0,0,0,0 ],[ 1,0,1,0,1,0,0 ]],
    > C2, [[ 0,0,0,0,0,0 ],[ 1,0,1,0,1,0 ]] );
    a (13,1024,1..13)1..2 blockwise direct sum code
  ------------------------------------------------------------------
  
  6.2-9 ConstructionXCode
  
  > ConstructionXCode( C, A ) ________________________________________function
  
  Consider  a  list of j linear codes of the same length N over the same field
  F,  C = C_1, C_2, ..., C_j, where the parameter of the ith code is C_i = [N,
  K_i,  D_i] and C_j subset C_j-1 subset ... subset C_2 subset C_1. Consider a
  list  of  j-1 auxiliary linear codes of the same field F, A = A_1, A_2, ...,
  A_j-1  where  the  parameter  of  the ith code A_i is [n_i, k_i=(K_i-K_i+1),
  d_i], an [n, K_1, d] linear code over field F can be constructed where n = N
  +  sum_i=1^j-1  n_i,  and d = min D_j, D_j-1 + d_j-1, D_j-2 + d_j-2 + d_j-1,
  ..., D_1 + sum_i=1^j-1 d_i.
  
  For more information on Construction X, refer to [SRC72].
  
  ---------------------------  Example  ----------------------------
    gap> C1 := BCHCode(127, 43);
    a cyclic [127,29,43]31..59 BCH code, delta=43, b=1 over GF(2)
    gap> C2 := BCHCode(127, 47);
    a cyclic [127,22,47..51]36..63 BCH code, delta=47, b=1 over GF(2)
    gap> C3 := BCHCode(127, 55);
    a cyclic [127,15,55]41..62 BCH code, delta=55, b=1 over GF(2)
    gap> G1 := ShallowCopy( GeneratorMat(C2) );;
    gap> Append(G1, [ GeneratorMat(C1)[23] ]);;
    gap> C1 := GeneratorMatCode(G1, GF(2));
    a linear [127,23,1..43]35..63 code defined by generator matrix over GF(2)
    gap> MinimumDistance(C1);
    43
    gap> C := [ C1, C2, C3 ];
    [ a linear [127,23,43]35..63 code defined by generator matrix over GF(2), 
      a cyclic [127,22,47..51]36..63 BCH code, delta=47, b=1 over GF(2), 
      a cyclic [127,15,55]41..62 BCH code, delta=55, b=1 over GF(2) ]
    gap> IsSubset(C[1], C[2]);
    true
    gap> IsSubset(C[2], C[3]);
    true
    gap> A := [ RepetitionCode(4, GF(2)), EvenWeightSubcode( QRCode(17, GF(2)) ) ];
    [ a cyclic [4,1,4]2 repetition code over GF(2), a cyclic [17,8,6]3..6 even weight subcode ]
    gap> CX := ConstructionXCode(C, A);
    a linear [148,23,53]43..74 Construction X code
    gap> History(CX);
    [ "a linear [148,23,53]43..74 Construction X code of", 
      "Base codes: [ a cyclic [127,15,55]41..62 BCH code, delta=55, b=1 over GF(2)\
    , a cyclic [127,22,47..51]36..63 BCH code, delta=47, b=1 over GF(2), a linear \
    [127,23,43]35..63 code defined by generator matrix over GF(2) ]", 
      "Auxiliary codes: [ a cyclic [4,1,4]2 repetition code over GF(2), a cyclic [\
    17,8,6]3..6 even weight subcode ]" ]
  ------------------------------------------------------------------
  
  6.2-10 ConstructionXXCode
  
  > ConstructionXXCode( C1, C2, C3, A1, A2 ) _________________________function
  
  Consider  a  set of linear codes over field F of the same length, n, C_1=[n,
  k_1, d_1], C_2=[n, k_2, d_2] and C_3=[n, k_3, d_3] such that C_2 subset C_1,
  C_3  subset  C_1  and C_4 = C_2 cap C_3. Given two auxiliary codes A_1=[n_1,
  k_1-k_2,  e_1]  and  A_2=[n_2,  k_1-k_3,  e_2]  over the same field F, there
  exists  an  [n+n_1+n_2,  k_1,  d]  linear  code C_XX over field F, where d =
  mind_4, d_3 + e_1, d_2 + e_2, d_1 + e_1 + e_2.
  
  The  codewords  of C_XX can be partitioned into three sections ( v;|;a;|;b )
  where v has length n, a has length n_1 and b has length n_2. A codeword from
  Construction XX takes the following form:
  
  --    ( v ; | ; 0 ; | ; 0 ) if v in C_4
  
  --    ( v ; | ; a_1 ; | ; 0 ) if v in C_3 backslash C_4
  
  --    ( v ; | ; 0 ; | ; a_2 ) if v in C_2 backslash C_4
  
  --    ( v ; | ; a_1 ; | ; a_2 ) otherwise
  
  For more information on Construction XX, refer to [All84].
  
  ---------------------------  Example  ----------------------------
    gap> a := PrimitiveRoot(GF(32));
    Z(2^5)
    gap> f0 := MinimalPolynomial( GF(2), a^0 );
    x_1+Z(2)^0
    gap> f1 := MinimalPolynomial( GF(2), a^1 );
    x_1^5+x_1^2+Z(2)^0
    gap> f5 := MinimalPolynomial( GF(2), a^5 );
    x_1^5+x_1^4+x_1^2+x_1+Z(2)^0
    gap> C2 := CheckPolCode( f0 * f1, 31, GF(2) );; MinimumDistance(C2);; Display(C2);
    a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)
    gap> C3 := CheckPolCode( f0 * f5, 31, GF(2) );; MinimumDistance(C3);; Display(C3);
    a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)
    gap> C1 := UnionCode(C2, C3);; MinimumDistance(C1);; Display(C1);
    a linear [31,11,11]7..11 union code of
    U: a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)
    V: a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)
    gap> A1 := BestKnownLinearCode( 10, 5, GF(2) );
    a linear [10,5,4]2..4 shortened code
    gap> A2 := DualCode( RepetitionCode(6, GF(2)) );
    a cyclic [6,5,2]1 dual code
    gap> CXX:= ConstructionXXCode(C1, C2, C3, A1, A2 );
    a linear [47,11,15..17]13..23 Construction XX code
    gap> MinimumDistance(CXX);
    17
    gap> History(CXX);        
    [ "a linear [47,11,17]13..23 Construction XX code of", 
      "C1: a cyclic [31,11,11]7..11 union code", 
      "C2: a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)", 
      "C3: a cyclic [31,6,15]10..13 code defined by check polynomial over GF(2)", 
      "A1: a linear [10,5,4]2..4 shortened code", 
      "A2: a cyclic [6,5,2]1 dual code" ]
  ------------------------------------------------------------------
  
  6.2-11 BZCode
  
  > BZCode( O, I ) ___________________________________________________function
  
  Given  a  set  of  outer  codes  of the same length O_i = [N, K_i, D_i] over
  GF(q^e_i), where i=1,2,...,t and a set of inner codes of the same length I_i
  =  [n,  k_i,  d_i]  over  GF(q),  BZCode  returns a Blokh-Zyablov multilevel
  concatenated   code   with   parameter  [  n  x  N,  sum_i=1^t  e_i  x  K_i,
  min_i=1,...,td_i x D_i ] over GF(q).
  
  Note  that  the  set of inner codes must satisfy chain condition, i.e. I_1 =
  [n,  k_1,  d_1] subset I_2=[n, k_2, d_2] subset ... subset I_t=[n, k_t, d_t]
  where  0=k_0  < k_1 < k_2 < ... < k_t. The dimension of the inner codes must
  satisfy the condition e_i = k_i - k_i-1, where GF(q^e_i) is the field of the
  ith outer code.
  
  For more information on Blokh-Zyablov multilevel concatenated code, refer to
  [Bro98].
  
  6.2-12 BZCodeNC
  
  > BZCodeNC( O, I ) _________________________________________________function
  
  This  function  is  the  same as BZCode, except this version is faster as it
  does  not  estimate the covering radius of the code. Users are encouraged to
  use this version unless you are working on very small codes.
  
  ---------------------------  Example  ----------------------------
    gap> #
    gap> # Binary code
    gap> #
    gap> O := [ CyclicMDSCode(2,3,7), BestKnownLinearCode(9,5,GF(2)), CyclicMDSCode(2,3,4) ];
    [ a cyclic [9,7,3]1 MDS code over GF(8), a linear [9,5,3]2..3 shortened code, 
      a cyclic [9,4,6]4..5 MDS code over GF(8) ]
    gap> A := ExtendedCode( HammingCode(3,GF(2)) );;
    gap> I := [ SubCode(A), A, DualCode( RepetitionCode(8, GF(2)) ) ];
    [ a linear [8,3,4]3..4 subcode, a linear [8,4,4]2 extended code, a cyclic [8,7,2]1 dual code ]
    gap> C := BZCodeNC(O, I);
    a linear [72,38,12]0..72 Blokh Zyablov concatenated code
    gap> #
    gap> # Non binary code
    gap> #
    gap> O2 := ExtendedCode(GoppaCode(ConwayPolynomial(5,2), Elements(GF(5))));;
    gap> O3 := ExtendedCode(GoppaCode(ConwayPolynomial(5,3), Elements(GF(5))));;
    gap> O1 := DualCode( O3 );;
    gap> MinimumDistance(O1);; MinimumDistance(O2);; MinimumDistance(O3);;
    gap> Cy := CyclicCodes(5, GF(5));;
    gap> for i in [4, 5] do; MinimumDistance(Cy[i]);; od;
    gap> O  := [ O1, O2, O3 ];
    [ a linear [6,4,3]1 dual code, a linear [6,3,4]2..3 extended code,
      a linear [6,2,5]3..4 extended code ]
    gap> I  := [ Cy[5], Cy[4], Cy[3] ];
    [ a cyclic [5,1,5]3..4 enumerated code over GF(5),
      a cyclic [5,2,4]2..3 enumerated code over GF(5),
      a cyclic [5,3,1..3]2 enumerated code over GF(5) ]
    gap> C  := BZCodeNC( O, I );
    a linear [30,9,5..15]0..30 Blokh Zyablov concatenated code
    gap> MinimumDistance(C);
    15
    gap> History(C);
    [ "a linear [30,9,15]0..30 Blokh Zyablov concatenated code of",
      "Inner codes: [ a cyclic [5,1,5]3..4 enumerated code over GF(5), a cyclic [5\
    ,2,4]2..3 enumerated code over GF(5), a cyclic [5,3,1..3]2 enumerated code ove\
    r GF(5) ]",
      "Outer codes: [ a linear [6,4,3]1 dual code, a linear [6,3,4]2..3 extended c\
    ode, a linear [6,2,5]3..4 extended code ]" ]
  ------------------------------------------------------------------