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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(self, key, column_parent, column, consistency_level):
 """ Increment or decrement a counter. Parameters: - key - column_parent - column - consistency_level """

    self._seqid += 1
    d = self._reqs[self._seqid] = defer.Deferred()
    self.send_add(key, column_parent, column, consistency_level)
    return d