Each polygon is on a layer and consists of a list of contour points and a set of holes. Each hole is a list of its contour points. Limitations:
Simple poly with holes example: io_poly.pcb
If PCB_FLAG_CLEARPOLYPOLY is set, the polygon will have a clearance in any non PCB_FLAG_CLEARPOLYPOLY polygons in the same layer group.
Simple poly-clear-poly example: io_poly_clr.lht; there are two non-clearing polygons (left and bottom; grabbing them for move highlights their as-drawn shape) and a rotated clearing polygon that clears into both.