Does TrillSpanner support text-spanner-interface fully?

[Kieren MacMillan]

Hello, all --

Can anyone tell me why the following doesn't work (for any values of  
shorten-pair)?

%% CODE SNIPPET BEGINS
\version "2.9.7"
\relative {
  \once \override TrillSpanner #'shorten-pair = #'( 5 . 10 ) c 
\startTrillSpan d\stopTrillSpan\startTrillSpan e\stopTrillSpan f g a b c
}
%% CODE SNIPPET ENDS

The same tweak works with other Objects which support the text- 
spanner-interface.

Thanks!
Kieren.