SPRITE(V,[#]s1, [#]s2)
Returns the vector from sprite s1 to s2 in radians.
The angle is based on the clock so if s2 is above s1 on the screen then the
answer will be zero. This can be used on any pair of sprites that are visible. If
either sprite is not visible the function will return -1.
This is particularly useful after a collision if the programmer wants to make some differential decision based on where the collision occurred. The angle is calculated between the centre of each of the sprites which may of course be different sizes.