theorem Th17:
  for A being OSSubset of OU0 holds the Sorts of OU0 in OSSubSort( A)
proof
  let A be OSSubset of OU0;
  reconsider X = the Sorts of OU0 as Element of SubSort(A) by MSUALG_2:38;
  the Sorts of OU0 is OrderSortedSet of S1 by OSALG_1:17;
  then
  X in { x where x is Element of SubSort(A): x is OrderSortedSet of S1};
  hence thesis;
end;
