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|- 'not' p => q & X |- 'not' q implies X|- p
proof
  assume X|- 'not' p => q;
  then X |- 'not' q => p by Th74;
  hence thesis by CQC_THE1:55;
end;
