function renderStatic (
  index,
  isInFor
) {
  var cached = this._staticTrees || (this._staticTrees = []);
  var tree = cached[index];
  // if has already-rendered static tree and not inside v-for,
  // we can reuse the same tree.
  if (tree && !isInFor) {
    return tree
  }
  // otherwise, render a fresh tree.
  tree = cached[index] = this.$options.staticRenderFns[index].call(
    this._renderProxy,
    null,
    this // for render fns generated for functional component templates
  );
  markStatic(tree, ("__static__" + index), false);
  return tree
}
Runtime helper for rendering static trees.