I wrote:
> Decrementing could probably be done similarly, except that W is needed:
>
>                 decf    ptr
>                 movlw   0x01
>                 btfss   ptr,4
>                 subwf   ptr

Oops!  That was obviously supposed to be:

                 decf    ptr
                 movlw   0x10
                 btfss   ptr,4
                 subwf   ptr