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

theorem
  (( (p => q) '&' p ) '&' s ) => q in HP_TAUT
proof
  set P = (p => q) '&' p;
A1: P => q in HP_TAUT by Th36;
  (P '&' s) => P in HP_TAUT by Def10;
  hence thesis by A1,Th23;
end;
