theorem
  for f being PartFunc of X,Y st y in rng f ex x being Element of X st x
  in dom f & y = f.x
proof
  let f be PartFunc of X,Y;
  assume y in rng f;
  then ex x being object st x in dom f & y = f.x by FUNCT_1:def 3;
  hence thesis;
end;
