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
  X|- p => 'not' q & X |- q implies X|- 'not' p
proof
  assume X|- p => 'not' q;
  then X |- q => 'not' p by Th73;
  hence thesis by CQC_THE1:55;
end;
