theorem Th16:
  for A being OSSubset of OU0 holds OSSubSort(A) c= SubSort(A)
proof
  let A be OSSubset of OU0;
  let x be object;
  assume x in OSSubSort(A);
  then
  ex y being Element of SubSort(A) st x = y & y is OrderSortedSet of S1;
  hence thesis;
end;
