code: public InternalDocumentRevision bySequence(long sequence) {
          return sequenceMap.containsKey(sequence) ? sequenceMap.get(sequence).getData() : null;
      }
docs: <p>Returns a {@link InternalDocumentRevision} from this {@code DocumentRevisionTree} with
  a particular sequence number.</p>
  
  @param sequence sequence number of the {@code DocumentRevision}
  @return the {@code DocumentRevision} with the given sequence number,
  null if no {@code DocumentRevision} has the given sequence number.