How to abbreviate 'note with the same note an octave higher, parenthesized' in Lilypond?

Question

Currently I write lilypond code that looks like this:

\version "2.14.2"

P = #parenthesize

\relative c, {
  \clef bass 
    <c \P c'> <e \P e'> <g \P g'>2 <c, \P c'>4 <d \P d'> <e \P e'>2
}

where I repeatedly mean 'this note, together with the same note one octave higher, parenthesized'.

I'd like a way to abbreviate this, so that I can write something like this:

\version "2.14.2"

poct = ...

\relative c, {
  \clef bass 
  \poct c \poct e \poct g2 \poct c,4 \poct d \poct e2
}

As suggested in a helpful answer to an earlier question of mine, I have tried to use a music function, but there is no way I can get this to work. The closest I can get is

poct = #(define-music-function
     (parser location note)
     (ly:music?)
   #{
     << $note \transpose c c \parenthesize $note >>
   #})

but this uses << .. >> instead of < .. >, which does not render the way I want (and with warnings), and I have no idea why the \transpose c c actually transposes anything.

Finally, tangentially related, when experimenting with music functions I found it even impossible just to create a music function that mimicks \repeat unfold 2; the following jumps down an octave between the third and fourth c:

\version "2.14.2"

double = #(define-music-function
     (parser location note)
     (ly:music?)
   #{
     $note $note
   #})

\relative c, {
  \clef bass 
  \double c \double e \double g2 \double c,4 \double d \double e2
}

Answer 1

I just got this answer from David Kastrup, one of the developers of LilyPond:

---

The "octave transposition" happens because basically \transpose c c is the same as \absolute (as LilyPond does not apply \relative to transposed music).

\absolute is not necessary in connection with \repeat unfold: \repeat unfold knows how to deal with relative music.

Version 2.14.2 is awfully old. At any rate, there is currently issue http://code.google.com/p/lilypond/issues/detail?id=3673 in the LilyPond tracker, and the respective code can be found at https://codereview.appspot.com/30890043/diff/40001/scm/music-functions.scm

And there we get indeed into Scheme programming. The respective defmacro-public would likely work even in 2.14 though #{ #} might not work properly.

With that definition, your double definition would become:

double = #(define-music-function (parser location note) (ly:music?) (make-relative (note) note #{ $note $note #}))

and would then work in both absolute and relative mode. If the 2.14 #{ #} does not jibe with make-relative, writing (make-sequential-music (list (ly:music-deep-copy note) (ly:music-deep-copy note))) should serve as a Scheme replacement.

The original problem becomes easier to understand once you realize that it's just a simple code replacement, so \relative { \double c' } becomes \relative { c' c' } using different octaves. The make-relative macro will to the \relative operation only on a single note, and then paste the results into the music expression.