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 Th20:
  (p => (q => r)) in TAUT(A) & p => q in TAUT(A) implies p => r in TAUT(A)
proof
  assume (p => (q => r)) in TAUT(A);
  then (p => q) => (p => r) in TAUT(A) by Th19;
  hence thesis by CQC_THE1:46;
end;
