class SHAPE {
  point POINTS[];

  area() {
    // Calculate the area of any polygon.
    result = 0;
    foreach(c = 0; c < POINTS.length - 1; c++) {
      result += POINTS[c].x * POINTS[c+1].y
      result += POINTS[c].y * POINTS[c+1].x
    }
    result += POINTS[POINTS.length].x * POINTS[0].y
    result += POINTS[POINTS.length].y * POINTS[0].x
    result = result / 2
    return result;
  }
};

extend SHAPE with RECTANGLE {
    int width, height;
    // We can use "specialized" variables to speed up computation (bit like an index/cache)
   // We still need to update POINTS in the superclass.

    area() {
      return width * height;
    }

   // ... A list of constraints? If we can prove its a "rectangle" we can use these methods
   // If constraints are broken, decomposes to a SHAPE.
   // If its ALWAYS a rectangle, we can code eliminate and remove unnecessary "SHAPE" code.
}