theorem Th28:
  for f being Function of D,D9 st D in UN & D9 in UN holds f in UN
proof
  let f be Function of D,D9;
  assume D in UN & D9 in UN;
  then
A1: Funcs(D,D9) in UN by CLASSES2:61;
  f in Funcs(D,D9) by FUNCT_2:8;
  hence thesis by A1,ORDINAL1:10;
end;
