( Drop RSP until I-2 == INTERPRET. )
: EXIT!
    ['] INTERPRET       ( I )
    BEGIN               ( I )
    DUP                 ( I I )
    R> DROP I 2- @     ( I I a )
    = UNTIL
    DROP
;

( a -- a+1 c )
: C@+ DUP C@ SWAP 1+ SWAP ;