[metapost] Components of a Clipped Picture

[Troy Henderson]

Is it possible to inspect the components of a clipped picture?

clip currentpicture to ...
for tkn within currentpicture:
...
endfor;

loops through the components of the "pre-clipped" currentpicture.  In
particular, I'm trying to determine if clipping a picture leaves it
equivalent to nullpicture.

Is this possible?

Thanks in advances,

Troy Henderson