+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 that
+ '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 (or a symbolic link to any of the same);
+ * 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 there are
+two ways to customize the response:
+
+First, *match* stanzas with the type *notfound* will be matched
+against any request that would result in a 404 error. The filename for
+such matching is that of the last succesfully found component, which
+may be a directory, for example in case a name component could not be
+found in the real filesystem; or a file, for example in case a file
+was found, but not matched by any *match* stanzas.
+
+Otherwise, any request that would result in a 404 response but is
+matched by no *notfound* stanza is instead passed to a default handler
+named `.notfound`, which is handled internally in *dirplex* by
+default, but may be overridden just as any other handler may be in a
+`.htrc` file or by global configuration. Note, however, that any
+request not matched by a *notfound* stanza will not have the
+`X-Ash-File` header added to it.
+
+The built-in `.notfound` handler can also be used in *match* or
+*capture* stanzas (for example, to restrict access to certain files or
+directories).