Click the buttons on the left to add polyhedra. Click the buttons on the bottom to remove them.
Click & Drag in the central area to rotate the model. Release the mouse during the movement
to obtain a continuous rotation.
Press the v key to swap between perspective and parallel projection.
The new polyhedron is inserted into the innermost one (if any).
A button is disabled when the applet doesn't know how to include the related
polyhedron in the current innermost.