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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 add_member_to_list(self, username, listname, member_type="USER"):
 """ Add a member to an existing list. Args: username (str):
 The username of the user to add listname (str):
 The name of the list to add the user to member_type (str):
 Normally, this should be "USER". If you are adding a list as a member of another list, set this to "LIST", instead. """

        return self.client.service.addMemberToList(
            listname, username, member_type, self.proxy_id
        )