reserve A for QC-alphabet;
reserve p, q, r, s, t for Element of CQC-WFF(A);
reserve X for Subset of CQC-WFF(A);

theorem Th4: :: Identity law
  p => p in TAUT(A)
proof
  ('not' p => p) => p in TAUT(A) & p => ('not' p => p) in TAUT(A)
     by CQC_THE1:42,43;
  hence thesis by Th3;
end;
