;; the node where this continuation is used, if unique. This is always
;; null in :DELETED and :UNUSED continuations, and is never null in
;; :INSIDE-BLOCK continuations. In a :BLOCK-START continuation, the
;; the node where this continuation is used, if unique. This is always
;; null in :DELETED and :UNUSED continuations, and is never null in
;; :INSIDE-BLOCK continuations. In a :BLOCK-START continuation, the