code: function attacher(destination, options) {
    var settings
  
    if (destination && !destination.process) {
      settings = destination
      destination = null
    }
  
    settings = settings || options || {}
  
    if (settings.document === undefined || settings.document === null) {
      settings.document = true
    }
  
    return destination ? bridge(destination, settings) : mutate(settings)
  }
docs: Attacher. If a destination is given, runs the destination with the new mdast tree (bridge-mode). Without destination, returns the mdast tree: further plug-ins run on that tree (mutate-mode).