theorem Th15:
  for f being Function of 4,REAL ex a,b,c,d st f = (0,1,2,3)-->(a,b,c,d)
proof
  let f be Function of 4,REAL;
  reconsider a = f.0, b = f.1, c = f.2, d = f.3 as Element of REAL
       by XREAL_0:def 1;
  take a,b,c,d;
  dom f = {0,1,2,3} by CARD_1:52,FUNCT_2:def 1;
  hence thesis by FUNCT_4:144;
end;
