Hi all,

there is a problem in this kind of logic:

    (cond ((not ta) +1)
      ((not tb) -1)
      ((string-lessp ta tb) -1)
      ((string-lessp tb ta) +1))))

in that when both ta and tb are nil then they are arbitrarily sorted.
Since the agenda sorting strategy is lexicographic this logic
virtually invalidates any strategy that puts tag or alpha first, v.g.
'(tag-up priority-down).

I've attached a patch returning nil when both ta and tb are nil so as
to fallback to the next sorting criterion without favoring lhs nor
rhs.

AFAICS this only affects tag and alpha comparators.

Best regards
--
Carlos