document sublist-lens and add it to unstable/lens

unstable/lens/main.rkt

@@ -4,10 +4,14 @@
          "compound.rkt"
          "list.rkt"
          "hash.rkt"
-         "view-set.rkt")
+         "view-set.rkt"
+         "sublist.rkt"
+         )
 
 (provide (all-from-out "syntax.rkt"
                        "compound.rkt"
                        "list.rkt"
                        "hash.rkt"
-                       "view-set.rkt"))
+                       "view-set.rkt"
+                       "sublist.rkt"
+                       ))

unstable/lens/main.scrbl

@@ -14,3 +14,4 @@ this library being backwards-compatible.
 @include-section["list.scrbl"]
 @include-section["hash.scrbl"]
 @include-section["syntax.scrbl"]
+@include-section["sublist.scrbl"]

unstable/lens/sublist.scrbl

@@ -0,0 +1,13 @@
+#lang scribble/manual
+
+@(require lens/doc-util/main)
+
+@title{Sublist lenses}
+
+@defmodule[unstable/lens/sublist]
+
+@defproc[(sublist-lens [i exact-nonnegative-integer?] [j exact-nonnegative-integer?]) lens?]{
+@lenses-unstable-examples[
+  (lens-view (sublist-lens 1 4) '(0 1 2 3 4 5))
+  (lens-set (sublist-lens 1 4) '(0 1 2 3 4 5) '(a b c))
+]}