ok, I don't mind using the union. But your Path
struct implies a couple very important differences
from my interface.

You're implying random access to any path segment.
In my Path, once you add a segment, you can't
rewind and replace it, or change what's already
been written.

Secondly, you're making it thoroughly easy to
create paths that can't be rendered by postscript
or libart by allowing an arbitrary number of
control points to exist in BezierCurve.