introduced this bug just a little while ago, when *adding* internal error
checks).

vgetargskeywords:  Rewrote the section that crawls over the format string.
+ Added block comment so it won't take the next person 15 minutes to
  reverse-engineer what it's doing.
+ Lined up the "else" clauses.
+ Rearranged the ifs in decreasing order of likelihood (for speed).