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
  (p => (q => r)) in TAUT(A) & p => q in TAUT(A) & p in TAUT(A) implies
  r in TAUT(A)
proof
  assume (p => (q => r)) in TAUT(A) & p => q in TAUT(A);
  then p => r in TAUT(A) by Th20;
  hence thesis by CQC_THE1:46;
end;
