C A graphical interface
  
  Here we describe a Tcl/Tk interface to specify numerical semigroups.
  
  
  C.1 Graphical interface
  
  C.1-1 XNumericalSemigroup
  
  > XNumericalSemigroup( [s] ) _______________________________________function
  
  This  function  pops  up  a  Tcl/Tk window which allows the specification of
  numerical semigroups. The optional argument s is a numerical semigroup whose
  definition becomes displayed in the graphical interface.
  
  The  interface is very simple, it allows to specify a numerical semigroup by
  its  generators  or  a  (proportionally)  modular numerical semigroup by its
  (proportionally)  modular condition. The GAP variable by which the semigroup
  will be known must also be specified.
  
  The  interface  also allows to apply some functions to the semigroup, namely
  AperyListOfNumericalSemigroupWRTElement                             (3.1-5),
  DrawAperyListOfNumericalSemigroup (3.1-6), SmallElementsOfNumericalSemigroup
  (3.1-3),                  GapsOfNumericalSemigroup                  (3.3-1),
  FrobeniusNumberOfNumericalSemigroup                (3.2-1)               and
  MinimalPresentationOfNumericalSemigroup   (4.1-2).   Finally,   it  is  also
  possible to add functions to this list and remove them.
  
  ---------------------------  Example  ----------------------------
    gap> s:=RandomNumericalSemigroup(3,9);
    <Numerical semigroup with 3 generators>
    gap> XNumericalSemigroup(s);
  ------------------------------------------------------------------
  
  This would pop up the following window: