S0          Returns address of PSP TOS. When PSP is empty,
            'S == S0                                 
PICK        Pick nth item from stack. "0 PICK" = DUP,
            "1 PICK" = OVER.
ROLL        Rotate PSP over n items. "1 ROLL" = SWAP,
            "2 ROLL" = ROT. 0 is noop.


