( returns negative value on error )
: _            ( c -- n )
    DUP '0' '1' =><= IF '0' - EXIT THEN
    DROP -1 ( bad )
;
: _pb          ( a -- n f, parse binary )
    ( '0': ASCII 0x30 'b': 0x62 0x6230 )
    DUP 1+ @ 0x6230 = NOT IF 0 EXIT THEN ( a 0 )
    ( We have "0b" prefix )
    DUP C@ ( a len )
    0 SWAP 1+ ( len+1 ) 3 DO ( a r )
        OVER I + C@ ( a r c ) _ ( a r n )
        DUP 0< IF 2DROP 0 UNLOOP EXIT THEN
        SWAP 1 LSHIFT + ( a r*2+n ) LOOP
    NIP 1 ;