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

val lens : view:('s -> 'a) -> set:('b -> 's -> 't) -> ('s, 't, 'a, 'b) lens
val view : ('s, 't, 'a, 'b) lens -> 's -> 'a