////   This file contains the C++ code from Program 12.20 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C++"//   by Bruno R. Preiss.////   Copyright (c) 1998 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.pads.uwaterloo.ca/Bruno.Preiss/books/opus4/programs/pgm12_20.cpp//void PartitionAsForest::CheckArguments (    PartitionTree const& s, PartitionTree const& t){    if (!IsMember (s) || s.parent != 0 ||	!IsMember (t) || t.parent != 0 || s == t)	throw invalid_argument ("incompatible sets");}void PartitionAsForest::Join (Set& s, Set& t){    PartitionTree& p = dynamic_cast<PartitionTree&> (s);    PartitionTree& q = dynamic_cast<PartitionTree&> (t);    CheckArguments (p, q);    q.parent = &p;    --count;}