is (context-has-engraver? ctx sym) possible?

[Mark Polesky]

This question is already buried in this post:
http://lists.gnu.org/archive/html/lilypond-devel/2009-08/msg00154.html

I'm starting a new thread to make it more visible.

Is it possible to write a scheme procedure to test if a given
context contains a particular engraver? Like...
(context-has-engraver? context 'Span_arpeggio_engraver)

This would have to return #t if a non-default engraver has been
added with \consists, and #f if a default engraver has been
removed with \remove. Or else it would be pointless.

Thanks.
- Mark