docs: function createBundler(opts, file, transform) {
    // omit file contents to make browserify-incremental work propery
    // on main entry (#4)
    opts.entries = file.path
    opts.basedir = path.dirname(file.path)
  
    let bundler = bundlers[file.path]
  
    if (bundler) {
      bundler.removeAllListeners('log')
      bundler.removeAllListeners('time')
    }
    else {
      bundler = browserify(Object.assign(opts, incremental.args))
      // only available via method call (#25)
      if (opts.external) {
        bundler.external(opts.external)
      }
      incremental(bundler)
      bundlers[file.path] = bundler
    }
  
    bundler.on('log', message => transform.emit('log', message))
    bundler.on('time', time => transform.emit('time', time))
  
    return bundler
  }