Lighttpd

I refer specifically to the section about "ways to substitute URI parts without needing a regex match"

I've tried several ways of using them, but the rewrite never works.

Here is one simple example.

example.conf

server.modules = ( "mod_rewrite" )

server.document-root = "." 

url.rewrite-once = (
    "/a" => "${url.path}" 
)

debug.log-condition-handling = "enable" 
debug.log-request-header   = "enable" 
debug.log-response-header  = "enable" 
debug.log-request-handling = "enable" 
debug.log-file-not-found   = "enable" 

/a

Anything

Sever started with

sudo lighttpd -D -f example.conf

Navigating to localhost/a results in "404 - Not Found"

In the logs I see

2018-09-05 22:10:27: (response.c.350) -- splitting Request-URI 
2018-09-05 22:10:27: (response.c.351) Request-URI     :  /a 
2018-09-05 22:10:27: (response.c.352) URI-scheme      :  http 
2018-09-05 22:10:27: (response.c.353) URI-authority   :  localhost 
2018-09-05 22:10:27: (response.c.354) URI-path (raw)  :  /a 
2018-09-05 22:10:27: (response.c.355) URI-path (clean):  /a 

The URI-path is "/a", so I thought "${url.path}" would also be "/a"
But the logs show it is mapped to the literal string "${url.path}".

For this simple little example, how would I use ${url.path} to map /a to /a?
Or, if this isn't how it should be used, in general, how do these similar replacements work?

Thank you for your consideration.

Ubuntu 08.04

lighttpd/1.4.45 (ssl) - a light and fast webserver
Build-Date: Feb 6 2018 12:41:51

Chrome
Version 68.0.3440.106 (Official Build) (64-bit)