Index: code-docs/pollen.scrbl
==================================================================
--- code-docs/pollen.scrbl
+++ code-docs/pollen.scrbl
@@ -187,26 +187,26 @@
     ◊say["Tavi"]{You also write fiction, or you used to. Do you still?}
     ◊say["Lorde"]{The thing is, when I write now, it comes out as songs.}
   }
 }|
 
-@defproc[(index [heading string?] [elements xexpr?] ...) txexpr?]
+@defproc[(index [#:key key string? ""] [elements xexpr?] ...) txexpr?]
 
-Creates an entry in the keyword index under @racket[_heading] that points back to this spot in the
-document. If @racket[_elements] is not empty, the web edition of the document will use it as the
-contents of an understated hyperlink to back to @racket[_heading] in the keyword index.
+Creates a bidirectional link between this spot in the document and an entry in the keyword index
+under @racket[_key]. If @racket[_key] is not supplied, the string contents of @racket[_elements] are
+used as the key.
 
 The example below will create two index entries, one under the heading “compassion” and one under
 the heading “cats”:
 
 @codeblock|{
   #lang pollen
 
   “I have a theory which I suspect is rather immoral,” Smiley 
   went on, more lightly. “Each of us has only a quantum of 
-  ◊index["compassion"]{compassion}. That if we lavish our concern
-  on every stray ◊index["cats"] cat we never get to the centre of 
+  ◊index{compassion}. That if we lavish our concern on every
+  stray ◊index[#:key "cats"]{cat} we never get to the centre of 
   things. What do you think of it?” 
 }|
 
 @defproc[(note [#:date date-str non-empty-string?]
                [#:author author string? ""]

Index: pollen.rkt
==================================================================
--- pollen.rkt
+++ pollen.rkt
@@ -94,11 +94,11 @@
 (poly-branch-tag section)
 (poly-branch-tag subsection)
 (poly-branch-tag code)
 (poly-branch-tag dialogue)
 (poly-branch-tag say)
-(poly-branch-tag index)
+(poly-branch-kwargs-tag index)
 (poly-branch-tag figure)
 (poly-branch-tag figure-@2x)
 (poly-branch-kwargs-tag blockcode)
 (poly-branch-kwargs-tag verse)          ; [#:title ""] [#:italic "no"]
 

Index: tags-html.rkt
==================================================================
--- tags-html.rkt
+++ tags-html.rkt
@@ -102,16 +102,17 @@
   (define file (or (assoc 'filename attrs) ""))
   (define codeblock `(pre [[class "code"]] (code ,@elems)))
   (cond [(string>? file "") `(@ (div [[class "listing-filename"]] 128196 " " ,file) ,codeblock)]
         [else codeblock]))
 
-(define (html-index . elems)
-  `(a [[id ,(here-id (list "_idx-" (uri-encode (car elems))))]
-       [href ,(string-append "/keyword-index.html#" (uri-encode (string-downcase (car elems))))]
-       [data-index-entry ,(car elems)]
+(define (html-index attrs elems)
+  (define index-key (maybe-attr 'key attrs (tx-strs `(span ,@elems))))
+  `(a [[id ,(here-id (list "_idx-" (uri-encode index-key)))]
+       [href ,(string-append "/keyword-index.html#" (uri-encode (string-downcase index-key)))]
+       [data-index-entry ,index-key]
        [class "index-link"]]
-      ,@(cdr elems)))
+      ,@elems))
 
 (define (html-say . elems)
   `(@ (dt ,(car elems) (span [[class "x"]] ": ")) (dd ,@(cdr elems))))
 
 (define (html-verse attrs elems)