Access a stem’s notehead

[Simon Albrecht]

Hello,

I want to write a before-line-breaking callback for stem and need the 
staff-position of the associated NoteHead.  (There aren’t going to be 
any chords.)  This
%%%%%
\version "2.19.35"
{
  \override Stem.before-line-breaking =
  #(lambda (grob) (format #t "~a\n"
                   (ly:grob-parent grob 0)))
  b'4
}
%%%%%
tells me that the parent is the NoteColumn. How do I retrieve the 
NoteHead from the Column?

TIA, Simon