Completion_heads_engraver flaws [was: Completion_heads_engraver doesn't play nicely with lyric alignment]

[Alexander Kobel]

Hi all,

I found another issue while trying to wrap my head around when exactly 
the completionBusy and melismaBusy should be set.

In the following MWE, the positioning of slurs over notes splitted by 
the Completion_heads_engraver lacks something. (See the attached image.)
Obviously, there are things that it cannot be reasonably expected to 
cope with (e.g., making no completion on staccati, or pushing a fermata 
to the last note head), but slurs seem like a common thing in, e.g., 
baroque music where I expect a use for the Completion_heads_engraver.

(For "PhrasingSlur"-like, I refer to 
https://music.stackexchange.com/questions/56941/should-slurs-encompass-tied-notes-from-before-and-or-after 
which is non-authoritative, but the accepted answer somewhat matches my 
intuition. I typically prefer the first variant for clarity, though, if 
space allows.)


\version "2.19.82"

\paper { ragged-last = ##f }

\markup "e.g., Boosey & Hawkes (PhrasingSlur-like); also recommended in 
Gould p. 62"
\score {
  { b2 b2(~ b2 a2~ a2 b2~ b2) b2 }
}

\markup "also correct, perhaps better for non-vocal music (personal 
opinion)?"
\score {
  { b2 b2~ b2( a2~ a2 b2)~ b2 b2 }
}

\markup "half-baked output with Completion_heads_engraver..."
\score {
  { b2 b1( a1 b1) b2 }
  \layout {
    \context {
      \Voice
      \remove Note_heads_engraver
      \consists Completion_heads_engraver
    }
  }
}


I presume that means that I'll have to resort to two different sources 
for Mensurstriche and "modern" layouts, but I mention it anyway for the 
sake of completeness.


Cheers,
Alex

completion-of-slurs.png
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