: (parseb)          ( a -- n f )
    ( '0': ASCII 0x30 'b': 0x62 0x6230: 25136 )
    DUP @ 25136 = NOT IF 0 EXIT THEN    ( a 0 )
    ( We have "0b" prefix )
    DUP 1- C@ ( a len )
    0 SWAP ( len ) 2 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 ;