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 Th74:
  X|- 'not' p => q implies X|- 'not' q => p
proof
  assume
A1: X|- 'not' p => q;
  X|- ('not' p => q) => ('not' q => p) by CQC_THE1:59;
  hence thesis by A1,CQC_THE1:55;
end;
