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

theorem
  (q => s) => (( p '&' q ) => s) in HP_TAUT
proof
  set P = p '&' q;
A1: P => q in HP_TAUT by Def10;
  (P => q) => ((q => s) => (P => s)) in HP_TAUT by Th21;
  hence thesis by A1,Def10;
end;
