{"ast_node_type":"other","tokens":[{"end":75,"kind":"keyword","start":0,"text":"//"}]}
    // see that, imagine that you have `P1: R` -- in that case, `R` must be
    // `R2` must be less than the universe of `R1` (i.e., `R1` flows `R2`). To
    // either the placeholder `P1` or the empty region in that same universe.