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

theorem
  ( p '&' (s '&' q) ) => ( p '&' (q '&' s) ) in IPC-Taut
proof
  set P = s '&' q, Q = q '&' s;
A1: P => Q in IPC-Taut by Th46;
  ( P => Q ) => (( p '&' P ) => ( p '&' Q )) in IPC-Taut by Th49;
  hence thesis by A1,Def14;
end;
