sentences:  If the ball on is a red, then by definition it cannot be snookered via another red, as it merely provides an alternative clean shot with another ball on.If the ball on is a red, and is snookered by a colour after a foul, then logically the red is either the final one or all reds are snookered by a colour ball, meaning the free ball has to be a colour.If the ball on is a colour ball that is snookered by a red, a previous red must have been successfully potted; the snooker therefore must be self-inflicted and cannot have occurred as the result of a foul.