Extended trills

[ben . tordoff]

Is it possible / easy to extend a trill over
several bars, with or without a preceding 'tr':

 tr~~~~~~~~~~~~~~~~~~~~
 O   | O   | O   | O   |
 |   | |   | |   | |   |

I seem to recall that in a certain other music
typesetting system this was done in the same way
as dynamics, but I can't find any reference to a
similar feature in the lily manual (apologies if
it is there and I've missed it). I've noticed that
there is now a \cresc \endcresc syntax for
typsetting 'cresc's, so is there a matching \trill
\endtrill ?

Thanks in advance

Ben

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