code: function(value) {
        // avoid to override the tag properties already set
        if (contains(keys, key)) return
        // check whether this value is an array
        isArr = isArray(value)
        // if the key was never set
        if (!value)
          // set it once on the tag instance
          parent[key] = dom
        // if it was an array and not yet set
        else if (!isArr || isArr && !contains(value, dom)) {
          // add the dom node into the array
          if (isArr)
            value.push(dom)
          else
            parent[key] = [value, dom]
        }
      }
docs: get the key value we want to add to the tag instance