Re: Issue 3918: Add \alternatingTimeSignatures (issue 97110045)

[Paul Morris]

Thomas Morley-2 wrote
> Maybe a step in this direction:
> 
> \version "2.19.6"
> 
> appear =
> #(define-music-function (parser location mrkp item)
>    (markup? symbol-list-or-music?)
>    (let ((stil (lambda (item)
>                  (grob-interpret-markup item mrkp))))
>      #{ \tweak stencil #stil #item #}))

Assuming we would want to support stencils as well as markups, we'd need a
new type predicate: 

markup-or-stencil?

And if it is to be as flexible as a \tweak, then it should also support
procedures (i.e. that return a stencil).  So we'd need: 

markup-or-stencil-or-procedure?

A stencil or procedure could just be passed through unmodified to the
\tweak, while a markup would be converted to a stencil first.  (Would there
be a way to get procedures that return a markup to work?  I don't think
so...)

http://lilypond.org/doc/v2.18/Documentation/notation/predefined-type-predicates

Cheers,
-Paul



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