/*
   plus(X,Y,Z) :- The sum of the numbers X and Y is Z.
*/
   plus(X,Y,Z) :- nonvar(X), nonvar(Y), Z is X + Y.
   plus(X,Y,Z) :- nonvar(X), nonvar(Z), Y is Z - X.
   plus(X,Y,Z) :- nonvar(Y), nonvar(Z), X is Z - Y.

%   Program 10.1   Multiple uses for plus