human
I am working on the file "Python/Chapter-07/2-3-Trees-Frame.ipynb".
The first task is:
The class Nil represents an empty tree.  It has no member variables of its own.
Can you write Python program for it?

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What is a the Python program you requested?
assistant
#Here is the Python program you requested.

class Nil(TwoThreeTree):
    def __init__(self):
        TwoThreeTree.__init__(self)
        
    def isNil(self):
        return True
    
    def __str__(self):
        return 'Nil()'