writing custom engravers in scheme - defining context properties?

[Maxwell Phillips]

Hello!

I've been in the process of switching over to LilyPond for the past several months and have been really impressed so far by quality of the results, as well as the huge amount of information available in the documentation and list-serv archives. That this is the first time I haven't been able to figure something out without posting here is amazing to me, so thank you all for your work!

I'm experimenting with writing custom scheme engravers, for example to deal with groups of staffs with complex rules about when each staff should be visible or hidden.

I can't figure out how to define the properties that engravers pass on to the contexts they "consist" (sorry if not using the right terminology).

For example, I want to define an engraver with property my-prop such that 

\new Staff \with { \consists My_engraver }

allows me to \set Staff.my-prop to change the behavior of My_engraver.

All the source code I can find for engraver definitions is in C++, not scheme, and the examples of scheme engravers I've found online don't seem to include user-settable properties. Is it possible to accomplish this?

Thanks,

Max