// Example:

void make_shape(CMesh& mesh)
{

        Poly poly1; // poly1 and poly2 will be added together.
        Poly poly2;
        Poly poly3; // After poly1 and poly2 are added, poly3 will be subtracted.
        
        // my_make_square(CPoly& poly, CVector origin, float size) is a custom function used to create a square with sides length size and centered at origin.
        
        my_make_square(poly1, CVector(-1.0, -1.0, 0.0), 2.0);
        my_make_square(poly2, CVector(+1.0, +1.0, 0.0), 2.0);
        my_make_square(poly3, CVector( 0.0,  0.0, 0.0), 1.0);   
        
        // Make sure the mesh is clear.
         
        mesh.clear();
        
        // Addition may yield more than one polygon. So we need a mesh to temporarily store the intermediate result, before we subtract poly3.

        CMesh temp;
        
        CCsg2D::I().addition(&poly1, &poly2, &temp);

        // Subtract poly3 from the intermediate results stored in temp. This is done by for every polygon in temp, calculating the subtraction of poly3.
        
        for (CPoly* poly = temp.polyHead; poly; poly = poly->next)
                CCsg2D::I().subtraction(poly, &poly3, &mesh);
        
}