Hello,
I found this link: http://oldsite.vislab.usyd.edu.au/users/manuals/sfx/houdini/vex/index.html [oldsite.vislab.usyd.edu.au], in a post on the Houdini Forum. The site says it was introduced for Houdini 4.0, Side Effects, Inc. 1999.
Is this information still valid for use with the current version of Houdini?
Best Regards,
Philip