code: function _join(pre, tail) {
      var _hasBegin = !!pre
      if(!_hasBegin) pre = ''
      if (/^\[.*\]$/.exec(tail)) return pre + tail
      else if (typeof(tail) == 'number') return pre + '[' + tail + ']'
      else if (_hasBegin) return pre + '.' + tail
      else return tail
  }
docs: append path to a base path