Direction of merged 'fa' shape note heads

Using property NoteCollision.fa-merge-direction, the direction of “fa” shape note heads (“fa”, “faThin”, etc.) can be controlled independently of the stem direction if two voices with the same pitch and different stem directions are merged. If this property is not set, the “down” glyph variant is used.

\version "2.24.0"

{
  \clef bass

  << { \aikenHeads
       f2
       \override Staff.NoteCollision.fa-merge-direction = #UP
       f2 }
  \\ { \aikenHeads
       f2
       f2 }
  >>
}