+A *match* stanza may also contain any number of the following,
+optional directives:
+
+*set* 'HEADER' 'VALUE'::
+
+ If the *match* stanza is selected as the match for a file, the
+ named HTTP 'HEADER' in the request is set to 'VALUE' before
+ passing the request on to the specified handler.
+
+*xset* 'HEADER' 'VALUE'::
+
+ *xset* does exactly the same thing as *set*, except the
+ 'HEADER' is automatically prepended with the `X-Ash-`
+ prefix. The intention is only to make configuration files
+ look nicer in this very common case.
+
+404 RESPONSES
+-------------
+
+A HTTP 404 response is sent to the client if
+
+ * The mapping procedure fails to find a matching physical file;
+ * A path element is encountered during mapping which, after URL
+ unescaping, either begins with a dot or contains slashes;
+ * The mapping procedure finds a file which is neither a directory nor
+ a regular file;
+ * An empty, non-final path element is encountered during mapping; or
+ * The mapping procedure results in a file which is not matched by any
+ *match* stanza.
+
+By default, *dirplex* will send a built-in 404 response, but any
+`.htrc` file or global configuration may define a request handler
+named `.notfound` to customize the behavior. Note that, unlike
+successful requests, such a handler will not be passed the
+`X-Ash-File` header.
+
+The built-in `.notfound` handler can also be used in *match* or
+*capture* stanzas (for example, to restrict access to certain files or
+directories).