;;; Boxed-Reg: any boxed register (any boxed object)
;;; Unboxed-Reg: any unboxed register (any unboxed object)
;;; Float-Reg, Double-Float-Reg: float in FP register.
-;;; Stack: boxed object on the stack (on cstack)
+;;; Stack: boxed object on the stack (on control stack)
;;; Word: any 32bit unboxed object on nstack.
;;; Double: any 64bit unboxed object on nstack.