theorem
  for f being Function st X1 c= X2 & Y1 c= Y2 holds <:f,X1,Y1:> c= <:f, X2,Y2:>
proof
  let f be Function;
  assume X1 c= X2 & Y1 c= Y2;
  then <:f,X1,Y1:> c= <:f,X2,Y1:> & <:f,X2,Y1:> c= <:f,X2,Y2:> by Th28,Th29;
  hence thesis;
end;
