code: public String getExtension(ProvFormat format) {
          String extension = UNKNOWN;
          if (format != null) {
              extension = extensionMap.get(format);
          }
          return extension;
      }
docs: Returns an extension for a given type of serialization of PROV
  @param format {@link ProvFormat} for which file extension is sought
  @return a String for the extension of a file containing such a serialization