code: boolean decorateAtFirst(byte[] decorator) {
          if (numTranscoders == 0) return decorateAt(decorator, 0);
          Transcoder transcoder = elements[0].transcoding.transcoder;
  
          if (!decorator(transcoder.source, transcoder.destination) && transcoder.compatibility.isDecoder()) {
              return decorateAt(decorator, 1);
          }
  
          return decorateAt(decorator, 0);
      }
docs: NOTE: Unused in MRI proper; for C exts, presumably