318: Mainline

The new version works for two reasons:

• If the value of self is 0, the recursivePowerOfTwo method answers 1.
• If the value of self is not 0, the recursivePowerOfTwo method first computes the result of the expression self - 1. Next, the recursivePowerOfTwo method sends the recursivePowerOfTwo message to that result to compute the power of 2 for a number that is 1 less than the value of self. Then, the recursivePowerOfTwo method may send another recursivePowerOfTwo message to ask itself to compute the power of 2 for a number that is 2 less than the original value of self, and so on, until the recursivePowerOfTwo method needs to deal with only 0.