
formulas(assumptions).

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

% Prover9 should produce a proof immediately.

  x + y = y + x.
  (x + y) + z = x + (y + z).
  ((x + y)' + (x + y')')' = x  # label(Robbins).

  exists c (c + c = c) # label(idempotent).

end_of_list.

formulas(goals).

  (x + y')' + (x' + y')' = y # answer(Huntington).

end_of_list.
