Create a custom articulation in LilyPond

Question

Could someone provide me with a minimal example of a new, custom articulation in LilyPond?

Let's say, for the sake of simplicity, that we use glyph "a" as the articulation, and let's call it \newArticulation.

Now I can use \markup as a workaround for creating something similar:

c'4^\markup{"a"}

but markup is not an articulation, and I would like to learn how to make a proper articulation, equivalent to the built-in ones, like \accent or \trill. Also, how would one make it turn upside down when it is below the note, like \fermata or \portato?

I've seen some examples on the LilyPond mailing list, but they seem to be very specific and mostly outdated.

Answer 1

\layout {
  \context {
    \Score
    scriptDefinitions =
    #(acons
      'a
      `((stencil . ,(lambda (grob)
                      (let ((d (ly:grob-property grob 'direction UP))
                            (mstc (grob-interpret-markup
                                   grob
                                   (markup #:override '(font-encoding . latin1)
                                           #:center-align
                                           "a"))))
                        (if (= d UP)
                            mstc
                            (ly:stencil-rotate
                             mstc 180 0 (interval-center (ly:stencil-extent mstc Y)))))))
        (direction . ,UP))
      default-script-alist)
  }
}

"a" =
#(make-articulation 'a)

{
  c' d'-\a e'^\a f'_\a
  e'' f''-\a g''^\a a''_\a
}