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

theorem
  for f being Function of X,X st f is one-to-one holds for x1,x2 st x1
  in X & x2 in X & f.x1 = f.x2 holds x1 = x2 by Th55;
