This subtemplate of {{BOOKNAME}} handles for {{BOOKNAME/core}} the case of userspace.

The two parameters expected are namespace and pagename.

At this writing, the associated book name for a page in userspace is everything before the second slash, except that if that ends with /sandbox, everything before the third slash is taken. Books and book fragments in userspace are likely to be exceptional anyway, so any default rule used here is only a guess and should probably be treated as provisional, subject to change without warning; but meanwhile, these rules were chosen as possibly-useful.