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 Th5:
  q => (p => q) in TAUT(A)
proof
  q => (('not' p => q) => q) in TAUT(A) & (q => (('not' p => q) => q)) => ((p
  => ('not' p => q)) => (q => (p => q))) in TAUT(A) by Lm2,Lm12;
  then p => ('not' p => q) in TAUT(A) & (p => ('not' p => q)) =>
(q => (p => q))
  in TAUT(A) by CQC_THE1:43,46;
  hence thesis by CQC_THE1:46;
end;
