reserve P,Q,X,Y,Z for set, p,x,x9,x1,x2,y,z for object;

theorem Th49:
  for y being object
  for f being Function of X,{y} st x in X holds f.x = y
proof let y be object;
  let f be Function of X,{y};
  x in X implies f.x in {y} by Th5;
  hence thesis by TARSKI:def 1;
end;
