Unwanted InstrumentName when using Dynamics in ChoirStaff

[Rafael Ramirez Morales]

While trying to use a single Dynamics definition as per PianoStaff in the
ChoirStaff a strange behaviour manifests:

The Dynamics "voice" gets labelled if an InstrumentName has been defined
for the PianoStaff in the a context within the general layout.

Here follow the MWE (a bit longer than strictly necessary, but illustrates
the problem better):

%% BEGIN MWE
\version "2.19.37" % also tried with the stable \version "2.18.2"

soprano = { s1 c''4  c''2 c''4 c''' c''8 c'' c''2 }
alto = \soprano
tenor = \soprano
bass = \soprano
piano = \soprano

dynamics = { s1 s4^\f s2\pp s4 s s8 s2 }

\layout {
  \context { \PianoStaff
             instrumentName = "Extra" }
}

\score {
  <<
    \new ChoirStaff <<
      \new Dynamics  = "dynamics" \dynamics
      \new Staff \with { instrumentName = "S." }{ \soprano }

      \new Dynamics  = "dynamics" \dynamics
      \new Staff \with { instrumentName = "A." }{ \alto }

      \new Dynamics  = "dynamics" \dynamics
      \new Staff \with { instrumentName = "T." }{\tenor }

      \new Dynamics  = "dynamics" \dynamics
      \new Staff \with { instrumentName = "B." }{ \bass }
    >>
    \new PianoStaff <<
      \new Dynamics  = "dynamics" \dynamics
      \new Staff \with { instrumentName = "Piano" }{ \piano }
    >>
  >>
}

%% END MWE

Adding the following hack in the \layout section *partially* solves the
problem.

%% BEGIN HACK

  %% This hack adds dynamics to the pedal group so it not be
  %% acknowledged by the Instrument_name_engraver
  %% cf.:
http://lists.gnu.org/archive/html/lilypond-user/2010-07/msg00414.html
  \override VerticalAxisGroup #'meta =
#(let* ((descr (assoc-get 'VerticalAxisGroup all-grob-descriptions))
        (meta (assoc-get 'meta descr))
        (ifaces (assoc-get 'interfaces meta)))
   ;; Adding piano-pedal-interface to this VerticalAxisGroup
   ;; prevents it being acknowledged by Instrument_name_engraver
   (acons 'interfaces (cons 'piano-pedal-interface ifaces)
          meta))

%% END HACK

Adding this block removes the spurious text on the ChoirStaff (as desired),
but leaves a double definition for the InstrumentName of the PianoStaff
(see overlapping names in the PianoStaff).

The hack is a functional workaround, but this does not seem to be a
desirable behaviour.