reusing metapost graphics

[Matthew Huggett <mhuggett@zam.att.ne.jp>]

Hi:

I'm designing a year's worth of English lessons for some Japanese kids, 
and I'm using Metapost and ConTeXt to generate flashcards for vocabulary 
training.  I have a few questions:
(1) I want to keep my Metapost figures defined in a ConTeXt environment 
file so that I can recall them as I need in future lessons.  Is there 
anything wrong with this approach?
(2) How do I make a Metapost figure defined in a ConTeXt file reusable? 
 Do I need to wrap the definition of the figure in a macro so that the 
figure is recreated everytime I need it?
(3) With \useMPgraphic, is there any mechanism to scale the image as 
with \useexternalfigure?

If this is all in the documentation, I apologize.  (I would appreciate a 
pointer to the documentation though :-) )