Assume that we have a sequence of 2-dimensional vertices V = (v2,v?..., vn] (n > 3), vi = (uten ) in an orthogonal coordinate system. Let M be the polygon surrounded by the edges constructed by connecting the adjacent vertices v, vi+1 (1 si sn - 1) as well as the last and first vertices vn, vl. The polygon M may not be convex. Assume that the edges meet only at their endpoints, and that vi + 2 holds for all i, j (1 3), vi = (uten ) in an orthogonal coordinate system. Let M be the polygon surrounded by the edges constructed by connecting the adjacent vertices v, vi+1 (1 si sn - 1) as well as the last and first vertices vn, vl. The polygon M may not be convex. Assume that the edges meet only at their endpoints, and that vi + 2 holds for all i, j (1