protected ExtendedCell getLowest(final ExtendedCell a, final ExtendedCell b) {
    if (a == null) {
      return b;
    }
    if (b == null) {
      return a;
    }
    return comparator.compareRows(a, b) <= 0 ? a : b;
  }
Returns Return lowest of a or b or null if both a and b are null
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/AbstractMemStore.java