code: def add(self, 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
docs: Increment or decrement a counter.
  
      Parameters:
       - key
       - column_parent
       - column
       - consistency_level