Undocumented feature? I think not...
The mechanics of passing a data source by reference via <include> was documented in /doc/acs-templating/tagref/include.html of a default ACS 4.2 install. I just checked and it looks like it hasn't been removed from the OpenACS docs either...
Which, of course, should not be interpreted as an argument against the need for more consistant documentation, just a clarification in this instance. 😉
