reserve i for object;
reserve S for non empty ManySortedSign;
reserve D for non empty set,
  n for Nat;

theorem Th4:
  for A being Universal_Algebra holds signature A is non empty
proof
  let A be Universal_Algebra;
  len(the charact of A) <> 0;
  then len(signature A) <> 0 by UNIALG_1:def 4;
  hence thesis;
end;
