reserve
   a,b,c,x,y,z,A,B,C,X,Y for set,
   f,g for Function,
   V for SetValuation,
   P for Permutation of V,
   p,q,r,s for Element of HP-WFF,
   n for Element of NAT;

theorem Th4:
  for f,g being one-to-one Function st f" = g" holds f = g
  proof
    let f,g be one-to-one Function;
    f"" = f & g"" = g by FUNCT_1:43;
    hence thesis;
  end;
