type ('s, 't, 'a, 'b) lens = ('s -> 'a) * ('b -> 's -> 't)
type ('s, 'a) lens' = ('s, 's, 'a, 'a) lens

let lens ~view ~set = view, set
let view (get, _) = get
let set (_, put) = put
let over (get, put) ~f source = put (f (get source)) source