2.19.15 slur accidental collision

[Paul Scott]

Hi,

I am trying to make a slur not collide with an accidental.  Reading the 
Learning Manual I find how to do:

  \override Slur.avoid-slur = #'inside
or
  \override Accidental.avoid-slur = #'inside
which doesn't seem to do the job.

>From the LM and the IR I don't follow how to set a value for 
accidental-collision.  Using the LyricText example I thought that 

  \override Slur.accidental-collision = #20 
would work but that gets an warning:

warning: cannot find property type-check for `accidental-collision' 
(backend-type?).  perhaps a typing error?

\version "2.19.15"

\relative{
  \override Slur.avoid-slur = #'inside
  \override Accidental.avoid-slur = #'inside
  \override Slur.accidental-collision = #20 
  \override Accidental.accidental-collision = #20 
  f'8( bes4)
}

TIA for any help with this,

Paul Scott