function addList(plugins) {
      let index = -1

      if (plugins === null || plugins === undefined) {
        // Empty.
      } else if (Array.isArray(plugins)) {
        while (++index < plugins.length) {
          const thing = plugins[index]
          add(thing)
        }
      } else {
        throw new TypeError('Expected a list of plugins, not `' + plugins + '`')
      }
    }
@param {PluggableList | null | undefined} plugins
@returns {undefined}