+<refsect2 id="hg"><title>Mercurial</title>
+
+<para>Under Mercurial, <application>irkerhook.py</application> can be
+invoked in two ways: either as a Python hook (preferred) or as a
+script.</para>
+
+<para>To call it as a Python hook, add the collowing to the
+"commit" or "incoming" hook declaration in your Mercurial
+repository:</para>
+
+<programlisting>
+[hooks]
+ incoming.irker = python:/path/to/irkerhook.py:hg_hook
+</programlisting>
+
+<para>When called as a script, the hook accepts a --repository option
+with value (the absolute pathname of the Mercurial repository) and can
+take a commit argument (the Mercurial hash ID of the commit or a
+reference to it). The default for the repository argument is the
+current directory. The default commit argument is '-1', designating
+the current tip commit.</para>
+
+<para>As for git, in both cases all variables may be set in the repo
+<filename>hgrc</filename> file in an [irker] section. Command-line
+variable=value arguments are accepted but not required for script
+invocation. No attempt is made to interpret an
+<filename>irker.conf</filename> file.</para>
+
+<para>The default value of the "project" variable is the basename
+of the repository directory. The default value of the "urlprefix"
+variable is the value of the "web.baseurl" config value, if it
+exists.</para>
+
+</refsect2>
+
+<refsect2 id="filter"><title>Filtering</title>
+
+<para>It is possible to filter commits before sending them to
+<application>irkerd</application>.</para>
+
+<para>You have to specify the <option>filtercmd</option> option, which
+will be the command <application>irkerhook.py</application> will
+run. This command should accept one arguments, which is a JSON
+representation of commit and extractor metadata (including the
+channels variable). The command should emit to standard output a JSON
+representation of (possibly altered) metadata.</para>
+
+<para>Below is an example filter:</para>
+
+<programlisting>
+#!/usr/bin/env python
+# This is a trivial example of a metadata filter.
+# All it does is change the name of the commit's author.
+#
+import sys, json
+metadata = json.loads(sys.argv[1])
+
+metadata['author'] = "The Great and Powerful Oz"
+
+print json.dumps(metadata)
+# end
+</programlisting>
+
+<para>Standard error is available to the hook for progress and
+error messages.</para>
+
+</refsect2>
+