code: public void addTokenRangeToCurrentBatchForCluster(String cluster, String placement, Collection<ScanRange> ranges) {
          PlanBatch batch = _clusterTails.get(cluster);
          if (batch == null) {
              batch = new PlanBatch();
              _clusterHeads.put(cluster, batch);
              _clusterTails.put(cluster, batch);
          }
          batch.addPlanItem(new PlanItem(placement, ranges));
      }
docs: Adds a collection of scan ranges to the plan for a specific placement.  The range collection should all belong
  to a single token range in the ring.
  @param cluster An identifier for the underlying resources used to scan the placement (typically an identifier for the Cassandra ring)
  @param placement The name of the placement
  @param ranges All non-overlapping sub-ranges from a single token range in the ring