Extracting parts: why does Devnull take as much space as a normal staff ?

[Jean-Charles Malahieude]

Hi all,


In the book I'm engraving, it happens that some instruments get "tacet" 
for certain pieces. In order to generate a full table of contents for 
the parts, I treat them through "Devnull". The problem is that this 
Devnull context takes as much vertical space as a normal staff of 
whatever kind I use (by the qay, a rhythmic staff is spread over the 
same extent as a 5 lines staff - min-dist = 8.00).
Using  annotate-spacing = ##t, Lily spits a
space: NaN/inf
min-dist: NaN/inf
padding: NaN/inf
bottom-of-extent: NaN/inf
extent-estimate: NaN/inf

---8<---
\version "2.14.0"

#(set-global-staff-size 20)
\paper { annotate-spacing = ##t
       ragged-bottom = ##t
       ragged-last-bottom = ##t }
\score { b'1}
\score {
%{ \new RhythmicStaff \with {
%  \override StaffSymbol   #'stencil = ##f
    fontSize = #-3
    \override StaffSymbol #'staff-space = #(magstep -3)
    \override StaffSymbol #'thickness = #(magstep -3) %}
\new Devnull  { s1 }

  \header { piece = "Piece two" opus = "TACET." }
  \layout { %#(layout-set-staff-size 1)
%{    \context {
      \RhythmicStaff
      \remove "Time_signature_engraver"
      \remove "Stem_engraver"
      \remove "Beam_engraver"
      \remove "Bar_engraver"
      \override VerticalAxisGroup
        #'default-staff-staff-spacing = #'((basic-distance . 0.01)
                                   (minimum-distance . 0.01)
                                   (padding . -24)
                                   (stretchability . 0))

    }
    \context {
      \Score
      \remove "Bar_number_engraver"
      \override VerticalAxisGroup
        #'default-staff-staff-spacing = #'((basic-distance . 0.01)
                                   (minimum-distance . 0.01)
                                   (padding . -24)
                                   (stretchability . 0))
  \override VerticalAxisGroup #'Y-extent = #'(-0.1 . 0)
    %}

  }
}
\score { b'1}
--->8---

After three full days digging in that; I'm really fed up and upset, and 
rely upon you.

The tracks I was following:
- have a piece and opus headers and a Devnull which normally should take 
no vertical space - wrong guess;
- have a piece and opus headers and a transparent staff I would 
translate up : impossible to change markup-score-spacing neither 
score-markup-spacing within a book(part);
- generate a transparent staff of ONE POINT HEIGHT: no way, it still 
takes the same vertical space as a regular one.
- create TACETpiece and TACETopus that would mimic the piece and opus 
headers whatever the customized layout I use for them: the only point of
entry I git grep are in Documentation/*

Thanks and sorry if for the disturbance,
Jean-Charles