how to shorten a TrillSpanner

[Kieren Richard MacMillan]

Hello, all!

I'm trying to fix the collision in the following example:

\version "2.9.5"
\relative
{
        \once \override TrillSpanner #'shorten-pair = #'( 10 . 10 )
                c1 \startTrillSpan b\stopTrillSpan\startTrillSpan 
c\stopTrillSpan
}

Doesn't matter what numbers I use -- no effect.
[n.b., I used the same technique on an OttavaBracket, and it worked  
as expected.]

Thanks,
Kieren.