Custom LilyPond spanners

Custom spanners to segment material in sketches.

   \new Staff {

      \override TextSpanner #'dash-fraction = ##f
      \override TextSpanner #'bound-details #'left 
         #'stencil-align-dir-y = #center
      \override TextSpanner #'bound-details #'left
         #'padding = #0.75
      \override TextSpanner #'bound-details #'right
         #'attach-dir = #right

      \override TextSpanner #'bound-details #'left
         #'text = \markup { \bold 7 }
      \override TextSpanner #'bound-details #'right
         #'text = \markup { \draw-line #'(0 . -1) }
      c'4 \startTextSpan
      c'4 \stopTextSpan

      \override TextSpanner #'bound-details #'left
         #'text = \markup { \bold 8 }
      c'4 \startTextSpan
      c'4 \stopTextSpan

   }

The settings to stencil-align-dir-y and attach-dir are tricky.