Need some guidance from a Lilypond Scheme expert

[Matthew Probst]

I understand the Scheme language at intermediate level, but am not adept at processing Lilypond data structures.

I'm looking into Snippet 465:

http://lsr.dsi.unimi.it/LSR/Item?id=465

I have the following in my Lilypond code, for typesetting a common guitar strum pattern:

rhtOne = #(rhythm-template #{ s16 s16 r8 s16 #} )

\relative c' { r4 \rhtOne <f a d>1 r2 }

This works fine.  However, this does not:

rhtOne = #(rhythm-template #{ s16 s16 r8 s16 #} )

Dnine = { \relative c' { <f a d>1 } }

\relative c' { r4 \rhtOne \Dnine r2 }

The problem is that the snippet constructs a single chord event to apply the rythm to, by walking down the 'elements property of the first parameter, and collecting their pitches into a list with (map):

http://pastebin.com/tSm1tw65

The data structure this sees from my first Lilypond example is like this:

http://pastebin.com/Tx8qhz6u

However, the data structure created by my second Lilypond example looks more like this:

http://pastebin.com/FuCRHDyK

In the first example, it sees an 'EventChord, so we properly fetch a list of all its children's 'pitch properties.  In the second example, it's 'SequentialMusic, not  an 'EventChord.  What I want to do is walk down the data structure and find the first EventChord or NoteEvent.

That doesn't seem too hard, actually.  Need to define a function first-non-sequence, and conditionally drill farther into the data structure until I find the first non-sequence.  Understanding that this is pseudocode instead of proper syntax,  I'd need to do something roughly like the following:

http://pastebin.com/3eNNYjHE

Also have to potentially recurse farther into nested levels of 'SequentialMusic or down each item in the 'elements list, I am comfortable fleshing it out to that level.

What I'm wondering, is what (if any) Lilypond scheme function do I use to determine the type of a music data structure?  I could certainly just say (car mus) but I'm wondering if there's a function I should use for the sake of encapsulation.  I see that one can get properties of a music element with (ly:music-property mus 'SYMBOL), the existing snippet code grabs the 'elements list using (ly:music-property mus 'elements).

If it's just a matter of use (car mus) and comparing the symbol against a given list of options, fine, I just don't know if I'm going the right way.  I've been looking at the docs for several days now, and have done so at several times in the past, and I'm just not quickly finding what I need.

My goal is to finally immerse myself into the Scheme side of Lilypond, and start implementing some fixes to the various rhythm-pitch template snippets that I've been wishing for, instead of complaining.