Thanks David, that makes a lot of sense! With your explanation in mind I’m thinking the current implementation in best.
Edward --- Dr. Edward Neeman www.neemanpianoduo.com > On 13 Sep 2018, at 9:42 pm, David Kastrup <d...@gnu.org> wrote: > > David Kastrup <d...@gnu.org <mailto:d...@gnu.org>> writes: > >> Edward Neeman <edward.nee...@gmail.com> writes: >> >>> Hello, >>> >>> I’ve come across some weird behaviour from the \stemNeutral command. This >>> doesn’t work: >>> >>> %%%% >>> >>> \version "2.19.82" >>> >>> \relative { \stemUp <g' bes d>8) r8 >>> \acciaccatura { \stemDown <ges' bes>8 } \stemUp <c e g!> r >>> \stemNeutral c,16 es ges bes } >>> >>> %%% >>> >>> Adding a second \stemNeutral command is necessary to reset the stems to >>> normal. Why might this be? >> >> Here is the deal. \acciaccatura does an implicit \temporary \stemUp (as >> that's a default for grace notes) which it cancels at the end of >> \acciacatura, provided that it is still active. >> >> \stemDown will cancel this preexisting \temporary \stemUp. >> Consequently, \acciacatura does not see the preexisting \temporary >> \stemUp at its end any more and will do nothing, in effect leaving a >> \temporary \stemDown on the stack. Your next \stemUp will replace it >> with a \temporary \stemUp which will get canceled by \stemNeutral, >> making the original \stemUp surface. >> >> Ugh. The solution is to cancel your \stemDown yourself, using >> \stemNeutral . >> >> Now let's assume that \acciaccatura would not do that kind of "matched >> pop" at its end and would remove the \stemDown. Now if you write >> \acciaccatura { \stemNeutral ... } this would pop the _preceding_ >> \stemUp which would be even worse. And was the reason the current >> behavior was implemented. >> >> So maybe the "matched pop" should be done in a different manner: if the >> previous state of the stack can be restored, do so, without actually >> looking at what you are going to pop in order to get there. > > The "matched pop" is also used for \once \override . If you write > something like > > \stemDown \once \stemUp \stemNeutral > > what's the expectation for the current timestep, and what's the > expectation for afterwards? > > In short, this is sort-of a messy area. It's not necessarily the best > way \acciaccatura works but the premise is that the behavior for its > private overrides is similar to \once \override and either try not to > tamper with stuff they don't understand because then the effects are > "non-local". > > Maybe the understanding could be extended beyond where it is now but it > would require some refactoring: the current implementation delivers a > "pop token" at negligible cost, and so there are no separate > implementations for when it is actually needed and not. For a slightly > better "pop token" like sketched above, there would be non-trivial > cost. I am not sure it would work all that much better. > > -- > David Kastrup
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