Overriding a pure print callback

[Nicolas Sceaux]

Hi,

I have in my tool box some overriden stencil functions, in particular
for bar lines (to define custom baroque repeat bars).

Now, some things seem to have changed since 2.15.20, with respect to
pure things, which show strange behavior when overriding the BarLine
stencil callback, with a function doing just the same thing:

\version "2.15.24"

#(define-public (bar-line::custom-print grob)
   (ly:bar-line::print grob))

\layout {
  \context {
    \Staff
    \override BarLine #'stencil = #bar-line::custom-print
  }
}

{ \key d \minor
  \time 3/4
  \set Score.skipBars = ##t
  R2.*8 \bar ":|" R2.*16 \bar "|." }

It seems that we now have to somehow register custom callbacks as being
something special, e.g. by patching scm/define-grobs to export symbol
`pure-print-callbacks' and doing:

#(set! pure-print-callbacks (cons bar-line::custom-print pure-print-callbacks))

Then, the above example works as expected:

My questions is, what is the proper way to define a custom callback for
e.g. the BarLine stencil property?

Nicolas