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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _waiting_expect(self):
        '''``True`` when the client is waiting for 100 Continue.
        '''
        if self._expect_sent is None:
            if self.environ.get('HTTP_EXPECT', '').lower() == '100-continue':
                return True
            self._expect_sent = ''
        return False