As before, open "stdprocs.e", online 6761 put the cursor just inside "pos(" and press alt-,
There are 3 popups: help, Symbol list, and symbol-help.
In the symbol help for "possiblePrefix", there is a hyper-link for "String Functions".
Click on that, and you get this:
[Window Title]
String_Functions.html
[Content]
Windows cannot find 'String_Functions.html'. Make sure you typed the name correctly, and then try again.
[OK]