code: function pauseStreams (streams, options) {
    if (!Array.isArray(streams)) {
      // Backwards-compat with old-style streams
      if (!streams._readableState && streams.pipe) streams = streams.pipe(PassThrough(options))
      if (!streams._readableState || !streams.pause || !streams.pipe) {
        throw new Error('Only readable stream can be merged.')
      }
      streams.pause()
    } else {
      for (let i = 0, len = streams.length; i < len; i++) streams[i] = pauseStreams(streams[i], options)
    }
    return streams
  }
docs: check and pause streams for pipe.