+
+ ;; Beware when changing this definition. NIL-the-symbol is defined
+ ;; using this layout, and NIL-the-end-of-list-marker is the cons
+ ;; ( NIL . NIL ), living in the first two slots of NIL-the-symbol
+ ;; (conses have no header). Careful selection of lowtags ensures
+ ;; that the same pointer can be used for both purposes:
+ ;; OTHER-POINTER-LOWTAG is 7, LIST-POINTER-LOWTAG is 3, so if you
+ ;; subtract 3 from (sb-kernel:get-lisp-obj-address 'NIL) you get the
+ ;; first data slot, and if you subtract 7 you get a symbol header.
+