Hints for Y1,Y2. * Begin by trying to define `X1` and `X2` directly [[assignment 4 hint 3 hint 1]], then once they're working, you can abstract back to a pair of combinators `Y1` and `Y2` that generate `X1` and `X2` for a given pair of arguments `T1` and `T2`. You want: X1 <~~> T1 X1 X2 and X2 <~~> T2 X1 X2 * X1 is presumably going to have some form like: (\elem1 elem2 ... (T1 (do-something-with-the-elems-to-make-X1) (do-something-with-the-elems-to-make-X2) )) some-elem1 some-elem2 ... Right? What will the form of X2 be? * Can you see how to go on? * Call the displayed abstract above `(\elem1 elem2 ... (...))` "elem0". What's the relation between `X1`, `elem0`, `elem1`, `elem2`...? * Go on, now you can do it.