% Predicates for simulating logical formula
false:-!,fail.

% The boolean predicate
boolean(true). boolean(false).
boolean(P,Q):-boolean(P),boolean(Q).
boolean(P,Q,R):-boolean(P,Q), boolean(R).

% definition of implication.
:-op(600,xfx,=>).
false=>true.
false=>false.
true=>true.

% definition of logical equivalence
:-op(600,xfx,iff).
true iff true.
false iff false.

:-op(600, yfx, and).
true and true.

:-op(600, yfx, or).
true or false.
false or true.
true or true.

pbool(P):-P, write('true     '), ! ;
             write('false    ').

truthtable(P):-boolean(P), write(P), nl,fail.
truthtable(P,Q):-boolean(P),pbool(P), write(Q), nl,fail.
truthtable(P,Q,R):-boolean(P),boolean(Q), pbool(P),pbool(Q), write(R), nl,fail.
truthtable(P,Q,R,S):-boolean(P),boolean(Q), pbool(P),pbool(Q),pbool(R), write(S), nl,fail.