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

theorem
  p '&' (q '&' s) => (p '&' q) '&' s in HP_TAUT
proof
A1: p '&' (s '&' q) => (p '&' q) '&' s in HP_TAUT by Lm19;
  p '&' (q '&' s) => p '&' (s '&' q) in HP_TAUT by Lm20;
  hence thesis by A1,Th23;
end;
