theorem Th6:
  ((p => q) => r) => (q => r) in TAUT(A)
proof
  q => (p => q) in TAUT(A) & (q => (p => q)) => (((p => q) => r) => (q => r))
  in TAUT(A) by Th1,Th5;
  hence thesis by CQC_THE1:46;
end;
