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 Th31:
  ('not' p => q) => ('not' q => p) in TAUT(A)
proof
  ('not' p => q) => ('not' q => 'not' 'not' p) in TAUT(A) & ('not' q => 'not'
  'not' p) => ('not' q => p) in TAUT(A) by Th26,Th29;
  hence thesis by Th3;
end;
