* "looks up" that int in the environment (i.e. modifies it)
* this is structurally parallel to the function `lookup` we used
* before to "look up" variables in the environment *)
(* double each leaf *)
let env = fun i -> i + i in
* "looks up" that int in the environment (i.e. modifies it)
* this is structurally parallel to the function `lookup` we used
* before to "look up" variables in the environment *)
(* double each leaf *)
let env = fun i -> i + i in