void clip(picture pic=currentpicture, path g, stroke=false,
pen fillrule=currentpen);
Clip the current contents of picture pic to the region bounded
by the path g, using fill rule fillrule (see fillrule).
If stroke=true, the clipped portion is the same as the region
that would be drawn with draw(pic,g,zerowinding); in
this case the path g need not be cyclic. While clipping has no notion
of depth (it transcends layers and even pages), one can localize clipping
to a temporary picture, which can then be added to pic.
For an illustration of picture clipping, see the first example in LaTeX usage.