docs: function isStream(io) {
    return typeof io === 'object' &&
      (
        // Readable stream.
        (typeof io.pipe === 'function' && typeof io.readable === 'boolean' &&
         io.readable) ||
        // Writable stream.
        (typeof io.write === 'function' && typeof io.writable === 'boolean' &&
         io.writable)
      );
  }