{"ast_node_type":"function","tokens":[{"end":75,"kind":"keyword","start":0,"text":"fn"}]}
    fn upmap_node_single(&self, input: &SyntaxNode) -> Option<SyntaxNode> {
    /// Follows the input one step along the syntax mapping tree
        let MappingEntry { parent, child_slot } = self.node_mappings.get(input)?;