reserve I,J for set,i,j,x for object,
  S for non empty ManySortedSign;

theorem
  for I be non empty set, S be non void non empty ManySortedSign, A be
MSAlgebra-Family of I,S, o be OperSymbol of S, x be set st x in rng (Frege (A?.
  o)) holds x is Function
proof
  let I be non empty set, S be non void non empty ManySortedSign, A be
  MSAlgebra-Family of I,S, o be OperSymbol of S, x be set;
  assume x in rng (Frege (A?.o));
  then ex y be object st y in dom (Frege (A?.o)) & (Frege (A?.o)).y = x by
FUNCT_1:def 3;
  hence thesis;
end;
