Hints for Y1,Y2.
-* Begin by trying to define `X1` and `X2` directly, 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`.
+* 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: