accidentalSugggestion

[E Appeldoorn]

I would like to keep the accidental created above the e! within the 
bounds of the larger slur (from f2 ending on e1)
This is what I came up with. The accidental is created but is kept above 
the slur. The internal documentation suggested to me this would work (it 
reads:  inside adjusts the slur if needed to keep the grob inside the 
slur.)
What have I missed?

  f2.(
  \set suggestAccidentals = ##t
  \override Staff.AccidentalSuggestion.avoid-slur = #'inside
  \override Staff.AccidentalSuggestion.font-size = #-2
  e!4~  |  e1)
Thanks in advance for your help,
Ursus