reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;

theorem
  p '&' (q '&' s) => (p '&' q) '&' s in IPC-Taut
proof
A1: p '&' (s '&' q) => (p '&' q) '&' s in IPC-Taut by Lm19;
  p '&' (q '&' s) => p '&' (s '&' q) in IPC-Taut by Lm20;
  hence thesis by A1,Th26;
end;
