$ cat > engine.py << EOF

  > 

  > from mercurial import templater

  > 

  > class mytemplater(object):

  >     def __init__(self, loader, filters, defaults):

  >         self.loader = loader

  > 

  >     def process(self, t, map):

  >         tmpl = self.loader(t)

  >         for k, v in map.iteritems():

  >             if k in ('templ', 'ctx', 'repo', 'revcache', 'cache'):

  >                 continue

  >             if hasattr(v, '__call__'):

  >                 v = v(**map)

  >             v = templater.stringify(v)

  >             tmpl = tmpl.replace('{{%s}}' % k, v)

  >         yield tmpl

  > 

  > templater.engines['my'] = mytemplater

  > EOF

  $ hg init test

  $ echo '[extensions]' > test/.hg/hgrc

  $ echo "engine = `pwd`/engine.py" >> test/.hg/hgrc

  $ cd test

  $ cat > mymap << EOF

  > changeset = my:changeset.txt

  > EOF

  $ cat > changeset.txt << EOF

  > {{rev}} {{node}} {{author}}

  > EOF

  $ hg ci -Ama

  adding changeset.txt

  adding mymap

  $ hg log --style=./mymap

  0 97e5f848f0936960273bbf75be6388cd0350a32b test