code: function(sel) {
    var cap, param;
  
    if (typeof sel !== 'string') {
      if (sel.length > 1) {
        var func = []
          , i = 0
          , l = sel.length;
  
        for (; i < l; i++) {
          func.push(parse(sel[i]));
        }
  
        l = func.length;
        return function(el) {
          for (i = 0; i < l; i++) {
            if (!func[i](el)) return;
          }
          return true;
        };
      }
      // optimization: shortcut
      return sel[0] === '*' 
        ? selectors['*'] 
        : selectors.type(sel[0]);
    }
  
    switch (sel[0]) {
      case '.': return selectors.attr('class', '~=', sel.substring(1));
      case '#': return selectors.attr('id', '=', sel.substring(1));
      case '[': cap = /^\[([\w-]+)(?:([^\w]?=)([^\]]+))?\]/.exec(sel);
                return selectors.attr(cap[1], cap[2] || '-', unquote(cap[3]));
      case ':': cap = /^(:[\w-]+)\(([^)]+)\)/.exec(sel);
                if (cap) sel = cap[1], param = unquote(cap[2]);
                return param ? selectors[sel](param) : selectors[sel];
      case '*': return selectors['*'];
      default:  return selectors.type(sel);
    }
  }
docs: Parsing
   parse simple selectors, return a `test`