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

theorem Th42:
  (q => s) => (( q '&' p ) => s) in IPC-Taut
proof
  set P = q '&' p;
A1: P => q in IPC-Taut by Def14;
  (P => q) => ((q => s) => (P => s)) in IPC-Taut by Th24;
  hence thesis by A1,Def14;
end;
