The Asymptote file
size(5cm);
draw(unitcircle ^^ expi(-3pi/4) -- expi(pi/4), red + linewidth(1cm) + opacity(0.5));
settings.outformat="pdf";
produces this
which is clearly not what was intended. I'd like to be able to draw (fill, etc.) several paths (or perhaps even an entire picture) that draw over each other as though they were opaque, but such that things drawn before the entire group are still visible (as in transparency). In TikZ, I can do this using transparency groups (although that feature has minor issues if you combine it with enough other esoteric features; see this question).
How can this be done using Asymptote?
