code: protected Entry getIrrelevantEntry(double threshold) {
          for (int i = 0; i < this.entries.length; i++) {
              Entry entry = this.entries[i];
              if (entry.isIrrelevant(threshold)) {
                  return entry;
              }
          }
  
          return null;
      }
docs: If there exists an entry, whose relevance is under the threshold given
  as a parameter to the tree, this entry is returned. Otherwise
  <code>null</code> is returned.
  @return An irrelevant <code>Entry</code> if there exists one,
  <code>null</code> otherwise.
  @see Entry
  @see Entry#isIrrelevant(double)