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