code: function expand (base, sub) {
      var children = splitSelector(sub)
      return splitSelector(base).reduce(function (selectors, parent) {
        return selectors.concat(children.map(function (child) {
          return parent + ' ' + child
        }))
      }, []).join(', ')
    }
docs: concats and regroups selectors. Deals with nested groups like  expand('#a, #b', '.x, .y')  returns:  '#a .x, #a .y, #b .x, #b .y'