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 `f` returns an `int
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 `f` returns an `int