Smart LilyPond context definition

LilyPond makes it easy to define new contexts with special attributes.

   \layout {
      \context {
         \Staff
         \type Engraver_group
         \name TwoLineStaff
         \alias Staff
         \override StaffSymbol #'line-positions = #'(-2 2)
      }
      \context {
         \Score
         \accepts TwoLineStaff
      }
   }

This layout block defines a special two-line staff.

   \new TwoLineStaff { c'4 d'4 e'4 f'4 }

Red and blue versions of our special TwoLineStaff are available like this.

   \layout {
      \context {
         \Staff
         \type Engraver_group
         \name TwoLineStaff
         \alias Staff
         \override StaffSymbol #'line-positions = #'(-2 2)
      }
      \context {
         \TwoLineStaff
         \type Engraver_group
         \name RedTwoLineStaff
         \alias TwoLineStaff
         \override NoteHead #'color = #red
         \override Stem #'color = #red
         \override Dots #'color = #red
      }
      \context {
         \TwoLineStaff
         \type Engraver_group
         \name BlueTwoLineStaff
         \alias TwoLineStaff
         \override NoteHead #'color = #blue
         \override Stem #'color = #blue
         \override Dots #'color = #blue
      }
      \context {
         \Score
         \accepts RedTwoLineStaff
         \accepts BlueTwoLineStaff
      }
   }

RedTwoLineStaff and BlueTwoLineStaff here extend TwoLineStaff in the manner of concrete classes extending an abstract base class.

   \new Score <<
      \new RedTwoLineStaff { c'4 d'4 e'4 f'4 }
      \new BlueTwoLineStaff { c'4 d'4 e'4 f'4 }
   >>

These definitions of RedTwoLineStaff and BlueTwoLineStaff avoid code duplication and let us have both types of staff in a single score. In a later post we'll look at a different way to define red and blue two-line staves for different editions of the same score.