code: function getItemId(keyAttr, originalItem, keyedItem, hasKeyAttrExpr) {
        if (keyAttr) {
          return hasKeyAttrExpr ?  tmpl(keyAttr, keyedItem) :  originalItem[keyAttr]
        }
  
        return originalItem
      }
docs: Return the value we want to use to lookup the postion of our items in the collection
  @param   { String }  keyAttr         - lookup string or expression
  @param   { * }       originalItem    - original item from the collection
  @param   { Object }  keyedItem       - object created by riot via { item, i in collection }
  @param   { Boolean } hasKeyAttrExpr  - flag to check whether the key is an expression
  @returns { * } value that we will use to figure out the item position via collection.indexOf