Description of \\ construct in NR

[Urs Liska]

Hi,

I stumbled over something I consider an omission in the NR but before
submitting a patch I'd like to clarify.

http://lilypond.org/doc/v2.19/Documentation/notation/multiple-voices#-The-double-backslash-construct
states that within the << \\ >> construct "temporary voices are created
and then discarded" for all music expressions.

But when two such constructs follow upon each other immediately
corresponding voices are continued. In the following snippet I would
have expected the ties to fail but they are printed nicely:

|{ << { a'1~ } \\ { r2 f'2~ } >> % c''1  << { a'1 } \\ { f'2 r2 } >> }|

This works if nothing is between. Putting a single common note between
the two blocks (uncommenting the c'') will break it. So I assume it is
not related to the fact that the voices are implicitly numbered "1", "2"
etc. but that when the << >> blocks follow immediately they are
implicitly joined to only one continuous Voice context.

The use case for this is generated/converted music that happens to come
"boxed" in these measure-based << >> blocks, and it would be an
inappropriate amount of work to modify the conversion to produce
continuous LilyPond-like voice variables.

So is my above assumption correct, is what I describe intended or at
least tolerated behaviour?

Urs

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