Creating a stencil from a markup

[Simon Albrecht]

Hello everybody,

I’d like to add a caesura sign on top of a barline by modifying the 
barline’s stencil. Here’s my attempt:

%%%%%%%%%%%%%%%%%%
\version "2.19.49"

{
  1
  \once\override BarLine.stencil =
  #(ly:stencil-combine-at-edge
     ly:bar-line::print
     1 ; y-axis
     1 ; on top
     (interpret-markup layout props (make-musicglyph-markup 
"scripts.caesura.curved"))
     2) %padding
  1
}
%%%%%%%%%%%%%%%%%%

However, layout is an unbound variable here (and so is props, I assume). 
How to do it correctly?

Best, Simon