
theorem Th1:
  for I be non empty set, M, N be ManySortedSet of I holds M +* N = N
proof
  let I be non empty set, M, N be ManySortedSet of I;
  dom M = I by PARTFUN1:def 2
    .= dom N by PARTFUN1:def 2;
  hence thesis by FUNCT_4:19;
end;
