Why is it that __sipath__ isn't set most of the time?

Docs say:

import siutils
# Add this plug-in path to python path
siutils.add_to_syspath(__sipath__)

but it errors! :(

#   File "<Script Block >", line 29, in <module>
#     siutils.add_to_syspath(__sipath__)
# NameError: name '__sipath__' is not defined
#

I'm trying to load a Python module in the same folder as the plugin. I
can't use Application.Plugins(PLUGIN_NAME).OriginPath until the plugin is
registered, and __sipath__ doesn't really work, so what else can I try?

Any help appreciated.
Cheers,

  -- Alan