syntax_data: {"ast_node_type":"function","tokens":[{"end":69,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn index_mut(&mut self, index: StackDepth) -> &mut Self::Output {
context_before: impl<X: Cx> IndexMut<StackDepth> for Stack<X> {
context_after:         &mut self.entries[index]