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

theorem
  ((p => q) => r) => (q => r) in HP_TAUT
proof
A1: (q => (p => q)) => (((p => q) => r) => (q => r)) in HP_TAUT by Th21;
  q => (p => q) in HP_TAUT by Def10;
  hence thesis by A1,Def10;
end;
