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
  p => 'not' q is valid implies q => 'not' p is valid
proof
  assume
A1: p => 'not' q is valid;
  (p => 'not' q) => (q => 'not' p) is valid;
  hence thesis by A1,CQC_THE1:65;
end;
