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 Th73:
  X|- p => 'not' q implies X|- q => 'not' p
proof
  assume
A1: X|- p => 'not' q;
  X|- (p => 'not' q) => (q => 'not' p) by CQC_THE1:59;
  hence thesis by A1,CQC_THE1:55;
end;
