private <T extends Map<?, Long>> Map<byte[], Long> flattenToLowestSequenceId(Map<byte[], T> src) {
    if (src == null || src.isEmpty()) {
      return null;
    }
    Map<byte[], Long> tgt = new HashMap<>();
    for (Map.Entry<byte[], T> entry : src.entrySet()) {
      long lowestSeqId = getLowestSequenceId(entry.getValue());
      if (lowestSeqId != HConstants.NO_SEQNUM) {
        tgt.put(entry.getKey(), lowestSeqId);
      }
    }
    return tgt;
  }
@return New Map that has same keys as <code>src</code> but instead of a Map for a value, it
        instead has found the smallest sequence id and it returns that as the value instead.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SequenceIdAccounting.java