;;; 2 = sbcl-0.6.4 uses COMPILE-OR-LOAD-DEFGENERIC.
;;; 3 = sbcl-0.6.6 uses private symbol, not :EMPTY, for empty HASH-TABLE slot.
;;; 4 = sbcl-0.6.7 uses HAIRY-DATA-VECTOR-REF and HAIRY-DATA-VECTOR-SET
;;; 2 = sbcl-0.6.4 uses COMPILE-OR-LOAD-DEFGENERIC.
;;; 3 = sbcl-0.6.6 uses private symbol, not :EMPTY, for empty HASH-TABLE slot.
;;; 4 = sbcl-0.6.7 uses HAIRY-DATA-VECTOR-REF and HAIRY-DATA-VECTOR-SET