code: function list(pid, options, callback) {
    if (typeof options === 'function') {
      callback = options;
      options = undefined;
    }
  
    if (typeof callback === 'function') {
      pidtree(pid, options, callback);
      return;
    }
  
    return pify(pidtree, pid, options);
  }
docs: Get the list of children pids of the given pid.
  @public
  @param  {Number|String} pid A PID. If -1 will return all the pids.
  @param  {Object} [options] Optional options object.
  @param  {Boolean} [options.root=false] Include the provided PID in the list.
  @param  {Boolean} [options.advanced=false] Returns a list of objects in the
  format {pid: X, ppid: Y}.
  @param  {Function} [callback=undefined] Called when the list is ready. If not
  provided a promise is returned instead.
  @returns  {Promise.<Object[]>} Only when the callback is not provided.