+ ;; a list of all the LAMBDA-VARs directly referred to from this
+ ;; function (or one of its LETs). This may include deleted variables
+ ;; because nobody bothers to clean them out.
+ ;;
+ ;; FIXME: This is completely analogous to the CALLS slot, except the
+ ;; elements here are LAMBDA-VARs instead of FUNCTIONALs. Maybe the
+ ;; two lists should be merged into a single list.
+ (refers-to-vars () :type list)