24. Pseudo lists
  
      |  Add(L,x) Let L be a pseudo list of length n, and x an object compatible with the entries in L. If x is not in L then this operation converts L into a pseudo list of length n+1 by adding x as the final entry. If x is in L the operation has no effect on L. | 
      |  Append(L,K) Let L be a pseudo list and K a list whose objects are compatible with those in L. This operation applies Add(L,x) for each x in K.                                                                                                                 | 
      |  ListToPseudoList(L) Inputs a list L and returns the pseudo list representation of L.                                                                                                                                                                           |