{"ast_node_type":"variable","tokens":[{"end":70,"kind":"keyword","start":0,"text":"let"}]}
                    let r1 = self.universal_regions.to_region_vid(r1);
                    // The bound says that `r1 <= r2`; we store `r2: r1`.
                    let r2 = self.universal_regions.to_region_vid(r2);