code: def replicate(self, doc_id, source, target, continuous=False):
          
          if doc_id in self["_replicator"]:
              return
          data = {
              "_id": doc_id,
              "source": source,
              "target": target,
              "continuous": continuous
          }
          self["_replicator"][doc_id] = data
docs: Starts a replication from the `source` database to the `target`
          database by writing a document with the id `doc_id` to the "_relicator"
          database