Instrument name placement

[Rune Zedeler]

There is something wrong with the routine that calculates the position
of the staff Instrument names.
In this example, too much space is added, and the name of the grand
staff is FAR out - above the top of the staff.

\version "1.3.129"
\include "paper20.ly"
\header{
        latexpackages = "amsmath";
}

\score{
  \notes <
  \context Staff = sa {\property Staff.instrument = "A" g,2 c''''2}
  \context Staff = sb {\property Staff.instrument = "B" g,2 c''''2}
  \context GrandStaff=gs <
    \property GrandStaff.instrument = "Gr"
    \context Staff = ga {\property Staff.instrument = "" c''1 }
    \context Staff = gb {\property Staff.instrument = "" c''1 }
  >
  \context Staff = sc {\property Staff.instrument =
"\small$\mathrm{\genfrac{}{}{0pt}{0}{C}{D}}$" <g'1 c''> }
  \context Staff = sd {\property Staff.instrument = "E" c''1 }

  >
  \paper{
    \translator { 
      \StaffContext
      \consists "Instrument_name_engraver";
    }
    \translator { 
      \GrandStaffContext
      \consists "Instrument_name_engraver";
    }

  }
}


-Rune