theorem
  for p,q being QC-formula of A holds p is closed & q is closed iff p <=> q
  is closed
proof
  let p,q be QC-formula of A;
  p <=> q = (p => q) '&' (q => p) by QC_LANG2:def 4;
  then p <=> q is closed iff p => q is closed & q => p is closed by Th22;
  hence thesis by Th26;
end;
