I found something somewhere that said something like [si]$avp(name) or whatever. What the heck does [si] mean???
Was there a colon after the "[si]"? If so, then I'll bet [si] was just a regular expression meaning "either the letter 's' or the letter 'i'". My limited understanding is that every AVP reference starts with either "i:" or "s:", depending on whether you're referencing it by its integer name or its string-format name. (The latter is more human-friendly, but slower.)
Tim