% FILE:  gv.pro
% TYPE:  Prolog source
% LINE:  very simple global variable ADT
% DATE:  November, 1995

  declare(Var,Val) :-
    assert(binding(Var,Val)).

  assign(Unbound,Bound) :-
    binding(Bound,Unbound).

  valueOf(Variable,Value) :-
    binding(Variable,Value).

  undeclare(Var) :-
    retract(binding(Var,_)).

  inc(Variable) :-
    retract( binding(Variable, Value)),
    NewValue is Value + 1,
    assert(binding(Variable, NewValue)).

  dec(Variable) :-
    retract(binding(Variable, Value)),
    NewValue is Value - 1,
    assert(binding(Variable, NewValue)).

  add(Variable,Number) :-
    retract( binding(Variable, Value)),
    NewValue is Value + Number,
    assert(binding(Variable, NewValue)).