fun rush(l1,[]) = []
|   rush([],l2) = []
|   rush(l1,l2) = (hd l1,hd l2) :: rush(l1, tl l2);

fun runrush(l1,[]) = []
|   runrush([],l2) = []
|   runrush(l1,l2) = rush(l1,l2) @ runrush(tl l1,l2);

runrush([1,4],[2,3]);