theorem :: (18)
  S.:^(R.:X) = (R*S`)`.:^X
proof
  (S.:^(R.:X))` = S`.:(R.:X) by Th48
    .= ((R*(S`)).:X) by RELAT_1:126;
  then S.:^(R.:X) = ((R*(S`)).:X)` .= (R*S`)`.:^X by Th49;
  hence thesis;
end;
