`val (-->) : 'a t -> key -> 'a`

`map-->key` returns the current binding of `key` in `map`, or raises `Not_found`. Equivalent to `find key map`.

`val (<--) : 'a t -> (key * 'a) -> 'a t`

`map <-- (key, value)` returns a map containing the same bindings as `map`, plus a binding of `key` to `value`. If `key` was already bound in `map`, its previous binding disappears. Equivalent to `add key value map`.