That is, we want to transform the ordinary tree `t1` (of type `int
tree`) into a reader object of type `(int -> int) -> int tree`: something
-that, when you apply it to an `int -> int` function returns an `int
-tree` in which each leaf `x` has been replaced with `(f x)`.
+that, when you apply it to an `int -> int` function `f` returns an `int
+tree` in which each leaf `x` has been replaced with `f x`.
With previous readers, we always knew which kind of environment to
expect: either an assignment function (the original calculator