anchor.diff:    ;; Pick the longest length.
     (sys.lap-x86:mov64 :rcx :rax)
     (sys.lap-x86:cmp64 :rax :rdx)
  -  (sys.lap-x86:cmov64g :rcx :rdx)
  +  (sys.lap-x86:cmov64ng :rcx :rdx)
     (sys.lap-x86:xor64 :rbx :rbx) ; offset
     (sys.lap-x86:xor64 :r11 :r11) ; CF save register.
     (sys.lap-x86:shl64 :rax 3)
  
positive.code:          (%%bignum-- x y))
          (t (error "Argument combination not supported."))))
  
  (defun generic-* (x y)
    (error "TODO"))
  
negative.code:   ;; Pick the longest length.
    (sys.lap-x86:mov64 :rcx :rax)
    (sys.lap-x86:cmp64 :rax :rdx)
    (sys.lap-x86:cmov64ng :rcx :rdx)
    (sys.lap-x86:xor64 :rbx :rbx) ; offset
    (sys.lap-x86:xor64 :r10 :r10) ; CF save register.
    (sys.lap-x86:shl64 :rax 3)
  