reserve T, X, Y for Subset of HP-WFF;
reserve p, q, r, s for Element of HP-WFF;

theorem Th26: :: Contraposition
  (p => (q => r)) => (q => (p => r)) in HP_TAUT
proof
A1: q => (p => q) in HP_TAUT by Def10;
  (p => (q => r)) => ((p => q) => (p => r)) in HP_TAUT by Def10;
  then
A2: (p => q) => ((p => (q => r)) => (p => r)) in HP_TAUT by Th20;
  ((p => q) => ((p => (q => r)) => (p => r))) => ((q => (p => q)) => (q =>
  ((p => (q => r)) => (p => r)))) in HP_TAUT by Th19;
  then
  (q => (p => q)) => (q => ((p => (q => r)) => (p => r))) in HP_TAUT by A2
,Def10;
  then (q => ((p => (q => r)) => (p => r))) in HP_TAUT by A1,Def10;
  hence thesis by Th20;
end;
