Centered Textspan for Piano music

[stefankaegi]

Good afternoon

I'd like to have a centered accelerando Textspan for a piano piece.
Something like this: acc. -   -   -   -

I'm using the last template from here:
http://lilypond.org/doc/v2.10/Documentation/user/lilypond/Piano-templates#Piano-templates

So I tried to do this Textspan in the dynamics section. It now looks
something like this:


> \override TextSpanner #'edge-text = #'("acc " . "")
> s2. s4\startTextSpan
> 
> s4.\stopTextSpan

Unfortunately this does not work. I hope you have an idea.

Thanks a lot for your effort
Cheers