(α, α) tree
The idea is that whatever type the free type variable `α` might be instantiated to,
-we will be a "type box" of a certain sort that "contains" values of type `α`. For instance,
+we will have a "type box" of a certain sort that "contains" values of type `α`. For instance,
if `α list` is our box type, and `α` is the type `int`, then in this context, `int list`
is the type of a boxed integer.