Frech trills and slurs. A bug?

[Francesco Spiga]

I have a problem which was also reported by Franz-Rudolf Kuhnen some 
times ago.

Lilypond places by default articulation symbols below the slurs.
So, if I try to mark a trill by a "+" (like in ancient French music), 
and above the note with trill there's a slur, the agrement is placed 
below the slur, instead of being above, as trills should be.

\version "2.11.51"

\score {

\relative c'' <<
\new Staff {

\time 2/4
b4( ais8._+) b16

>>
}

Is it possible to deplace the + above the slur? I tried the command 
\once \override Script #'padding - #*.* but nothing has changed.

Thank you
F.