code: public static synchronized void addExtractor(
          String name, DependencyExtractor extractor) {
          addExtractor(name, extractor, defaultExtractor == null);
      }
docs: Adds the provided {@link DependencyExtractor} to the set of available
  extractors.  If an existing extractor has the same name, the existing
  extactor is overwritten.  If no extractors have been added, the provided
  extractor becomes the default extractor.
  
  @param name the name of the parser that generated the parse trees.
  @param extractor the an extractor instance
  
  @throws NullPointerException if either {@code name} or {@code extractor}
  are {@code null}.