Compose lenses whilst preserving unfocussed data

fieldglass.mli

@@ -5,3 +5,4 @@ val lens : view:('s -> 'a) -> set:('b -> 's -> 't) -> ('s, 't, 'a, 'b) lens
 val view : ('s, 't, 'a, 'b) lens -> 's -> 'a
 val set : ('s, 't, 'a, 'b) lens -> 'b -> 's -> 't
 val over : ('s, 't, 'a, 'b) lens -> f:('a -> 'b) -> 's -> 't
+val compose : ('s, 't, 'a, 'b) lens -> ('a, 'b, 'c, 'd) lens -> ('s, 't, 'c, 'd) lens