reserve a, I for set,
  S for non empty non void ManySortedSign;

theorem Th18:
  for A being non-empty MSAlgebra over S, o being OperSymbol of S
  for x being Element of Args(o,A) holds Den(o,A).x in (the Sorts of A).(
  the_result_sort_of o)
proof
  let A be non-empty MSAlgebra over S, o be OperSymbol of S, x be Element of
  Args(o,A);
  Den(o,A).x is Element of (the Sorts of A).((the ResultSort of S).o) by
FUNCT_2:15;
  hence thesis;
end;
