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Once you see that you can define recursivePowerOfTwo in terms of powerOfTwo, you are ready to learn how to turn recursivePowerOfTwo into a recursive method that does not rely on powerOfTwo.

First, note that you can eliminate the need to call powerOfTwo in the simple case in which the value of recursivePowerOfTwo's receiver is 0:

Integer method definition • instance 
  self = 0 
    ifTrue: [^ 1] 
    ifFalse: [^ self powerOfTwo] 
Transcript show: 4 recursivePowerOfTwo printString; cr