provide and add docs

unstable/lens/define-nested.scrbl

@@ -0,0 +1,23 @@
+#lang scribble/manual
+
+@(require lens/private/doc-util/main)
+
+@title{Lenses for nested data}
+
+@defmodule[unstable/lens/define-nested]
+
+@defform[(define-nested-lenses [base-id base-lens-expr] clause ...)
+         #:grammar ([clause [sub-id sub-lens-expr]])]{
+A shorthand for defining composed lenses for nested data structures.
+
+For example, if there is a @racket[top] struct containing a
+@racket[middle] struct, which contains an @racket[x] field and a
+@racket[y] field, a form like:
+@(racketblock
+  (define-nested-lenses [top-middle top-middle-lens]
+    [x middle-x-lens]
+    [y middle-y-lens]))
+Will define @racket[top-middle-x-lens] and @racket[top-middle-y-lens]
+as @racket[(lens-thrush top-middle-lens middle-x-lens)] and
+@racket[(lens-thrush top-middle-lens middle-y-lens)].
+}

unstable/lens/main.rkt

@@ -1,5 +1,6 @@
 #lang reprovide
 "arrow.rkt"
+"define-nested.rkt"
 "dict-nested.rkt"
 "hash-filterer.rkt"
 "if.rkt"

unstable/lens/main.scrbl

@@ -13,6 +13,7 @@ this library being backwards-compatible.
 
 @(include-sections
   "arrow.scrbl"
+  "define-nested.scrbl"
   "dict-nested.scrbl"
   "hash-filterer.scrbl"
   "if.scrbl"