-Hints for enumerating a tree's fringe.
+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.
+