Prolog variable in input. Use lower-case or quote it
====================
Formulae should only contain non-logical ground terms (with lower-case letters for variables), the atoms 'true' and 'false', the operators 'not', 'and', 'or', 'xor', 'implies', 'iff' and parentheses.
The predicate tautology(Term) succeeds if Term represents a tautology. It
uses Wang's method of sequents to check Term.
*/
go :- propositions.
propositions :-
nl, nl, read(T), nl,
(
\+ ground(T) -> nl,
write('Prolog variable in input. Use lower-case or quote it.')