Pong: one of the originals and good for a second-forth

This version the two paddles go up and down together
by using the up/down cursors.

At the end of the lesson - hopfully most, if not all,
will have a working version - ask the class how
this game could be improved.

Rather disappointedly one gets ''change the background,
Add music, change the ball etc.'' It takes a bit of prompting
To get make the paddles move independently
for a two player game.