[Lilypond-auto] Issue 4083 in lilypond: \keepWithTag does not nest well

[lilypond]

Status: Accepted
Owner: ----
Labels: Type-Enhancement

New issue 4083 by address@hidden: \keepWithTag does not nest well
https://code.google.com/p/lilypond/issues/detail?id=4083

With multiple independent uses for tags, \keepWithTag becomes almost  
useless because it will delete material belonging to independent tags.  In  
particular when tags are used internally in music functions used for  
structuring a document, the interference can get annoying.

There would be several possibilities for reorganizing this.

a) Provide a declaration \tagLayer #'(divI divII unisono) that declares the  
given tags as belonging to one layer.  \keepWithTag will only remove  
material with tags in the same layer.  Should be pretty robust and fully  
backwards-compatible but relies on global side effects.

b) \keepWithTag only removes tagged music without the given tag(s) in  
sequential or parallel music only when one of the named tags is actually  
present on one of the other elements of the list.  That's probably my  
favorite though it will likely mandate that you place all the tags in one  
construct or via one function.

c) \keepWithTag, while keeping one piece of tagged music, removes all tags  
on the same music so that another call of \keepWithTag with a different tag  
will no longer see them.  That one seems complex to explain and would  
likely either require using \keepWithTag calls in a certain order (in->out  
or was it the other way round) or would use a "commutatable" mechanism that  
would likely be even harder to explain.

I've been toying with variants of c) mentally but while it would likely be  
more backwards-compatible than b) concerning single tag layers (which are  
pretty much the only thing that will work satisfactorily right now), it's  
probably too hard to explain.

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