tuplet slurs

[Werner LEMBERG]

For voices with lyrics it is common to put triplet indications always
above the staff, using the following rules.

   . stems up or down, no beam: as usual (i.e., a number and a bracket
     at the top, as if using \tupletUp)

   . stems up, with beam: as usual (i.e., a number over the beam)

The last case, however, is unusual:

   . stem down, with beam: a number and a *slur* at the top.

I would like to have a single command that makes lilypond do that
automatically.  Has this been requested before?  A quick searched
yielded nothing.

Hopefully, the attached images makes everything clear (note that I
don't need full brackets).


    Werner