theorem Th27:
  u => v = FinJoin(SUB u, (the L_meet of NormForm A).:(
  pseudo_compl(A), StrongImpl(A)[:](diff u, v)))
proof
  deffunc IMPL(Element of NormForm A, Element of NormForm A) = FinJoin(SUB $1,
  M(A).: (pseudo_compl(A), StrongImpl(A)[:](diff $1, $2)));
  u "/\" IMPL(u,v) [= v & for w st u "/\" w [= v holds w [= IMPL(u,v) by Lm9;
  hence thesis by FILTER_0:def 7;
end;
