Before: sprite 3 uses the `wait` block to wait for the other two sprites
to finish. After the `wait` blocks, sprite 3 sends "end" to the test. If
the sprites take too long to finish, they might not be done before the
"end" message.
After: sprite 3 sets a variable called `finished` to zero on startup,
and increments it every time it hears a new message. Each sprite sends
that new message once it's done. When sprite 3 notices that `finished >=
2`, it sends the "end" message.