code: public boolean addConverter(byte[] source, byte[] destination, int n) {
          if (started) return false;
          TranscoderDB.Entry entry = TranscoderDB.getEntry(source, destination);
          if (entry == null) return false;
          Transcoder transcoder = entry.getTranscoder();
          if (transcoder == null) return false;
          addTranscoderAt(transcoder, n);
          return true;
      }
docs: /* rb_econv_add_converter