then we start with S, and can choose a rule to apply to it. If we choose rule 1, we obtain the string aSb. If we then choose rule 1 again, we replace S with aSb and obtain the string aaSbb. If we now choose rule 2, we replace S with ba and obtain the string , and are done. We can write this series of choices more briefly, using symbols: S \Rightarrow aSb \Rightarrow aaSbb \Rightarrow aababb.
