[Therion] Metapost for a line ending in an ellipse

[Tarquin Wilton-Jones]

I am going to answer my own question.

> Is there maybe another way? Can I say "draw the line, erase the line
> within the area of this elliptical 'circle', draw this elliptical
> 'circle'", in that sequence?


Yes.

"unfill" is not useful. It actually fills with the background colour,
which looks wrong when opacity is <100, and it obscures other things
behind it.

"clip" is a useful function, but it can only apply slipping to show
items *within* a path, not outside it.

The answer is to clip showing the whole drawing area (a box from
-infinity->infinity in x and y planes), and part way around, come in to
the inner path (an ellipse in my case), and traverse it in the opposite
direction to the direction that the drawing area was traversed. Then go
back out to the edge of the drawing area on exactly the same path as was
used to come in to the inner path. Circles are natively drawn from the
right side, anticlockwise, to make sure to traverse the outer edges of
the drawing area clockwise:
 ________________
|        ___     |
|       (___)----|
|________________|

That was a fun learning exercise. Results will be on the wiki once I am
happy with my symbol.