Flassari/as3clipper — comment on commit d78208094f3ee97917e5a72b89e7b08ee00df732
File: alchemy/alchemywrapper.cpp line 65
By makc:

Doing single AddPolygon here basically prevents using the result of one clip as an input to another clip, if there's more than one sub-poly. Why not do AddPolygons instead? If you want you could do some voodoo on AS3 side to still accept [pt1, pt2, pt3] instead of [[pt1, pt2, pt3]] for simple polygons, but limiting it here severely cripples the library.