Get context in Scheme function (determining current moment)

[Urs Liska]

Hi,

when I'm trying to determine the the position in a piece when inside a 
music function I think I have to use either
ly:context-current-moment
or
ly:context-now

Both functions take a 'context' as argument, but I don't see where this 
should be taken from.

so
a)
how do I get a current context from when inside a Scheme function?
and
b)
how would I proceed to determine the position in a score when the Scheme 
function is called?

I don't know if I need a) for b) but wanted to ask for it anyway.

Best
Urs