assign(new_constants, 1).

% This lemma says that if there exists an idempotent
% elememt, then a Robbins algebra is Boolean.

formulas(sos).
x + y = y + x.
(x + y) + z = x + (y + z).
((x + y)' + (x + y')')' = x  # label(Robbins).
c + c = c.
end_of_list.

set(restrict_denials).

formulas(goals).
(x + y')' + (x' + y')' = y # answer(Huntington).
end_of_list.