How to locate all layout-property values for a particular LayoutObject in the source files?

[James]

Hello,

I apologise in advance if I ask this question in a less-than-meaningful way, 
simply because I don't know what the correct terms for some these 
LilyPond-specific things are (or I mix them up), but I hope you will get what 
it is I am asking for anyway.

If I take this example of an @lilypond example in the Ancient Music section:

---8<---

\override NoteHead.style = #'petrucci
a'\maxima a'\longa a'\breve a'1 a'2 a'4 a'8 a'16 a'
\override NoteHead.style = #'semipetrucci
a'\breve*5/6
\override NoteHead.style = #'blackpetrucci
a'8*4/3 a'

--->8---

Where would I go to see the 'complete list' of all possible NoteHead.style 
values - the 'values' (?) that come after the "#'" (I forget what they are 
called), if for example a developer added a style but hadn't put anything in 
the NR?

Let's say there was a NoteHead.style = #'semiblackpetrucci (I know there isn't 
such a thing) but which file would I see these defined?

grepping just for (say) 'blackpetrucci' get's me lots of hits in the font files 
and a few 'scm' files but all I want to see is the possible list of 
'NoteHead.styles' that can be defined within the source.

I do hope that made sense?

Thank you for your time.

James