code: public double areaSquared(HalfEdge hedge0, HalfEdge hedge1) {
          Point3d p0 = hedge0.tail().pnt;
          Point3d p1 = hedge0.head().pnt;
          Point3d p2 = hedge1.head().pnt;
  
          double dx1 = p1.x - p0.x;
          double dy1 = p1.y - p0.y;
          double dz1 = p1.z - p0.z;
  
          double dx2 = p2.x - p0.x;
          double dy2 = p2.y - p0.y;
          double dz2 = p2.z - p0.z;
  
          double x = dy1 * dz2 - dz1 * dy2;
          double y = dz1 * dx2 - dx1 * dz2;
          double z = dx1 * dy2 - dy1 * dx2;
  
          return x * x + y * y + z * z;
      }
docs: return the squared area of the triangle defined by the half edge hedge0
  and the point at the head of hedge1.
  
  @param hedge0
  @param hedge1
  @return