: walk ( stack point -- stack' )
    {
        [ clone [ 1 + ] change-x over push ]
        [ clone [ 1 - ] change-x over push ]
        [ clone [ 1 + ] change-y over push ]
        [ clone [ 1 - ] change-y over push ]
    } cleave ;

: ?walk ( total seen stack point -- total seen stack )
    dup walkable? [ [ 1 + ] 3dip walk ] [ drop ] if ;

: until-empty ( seq quot -- )
    [ dup empty? ] swap until drop ; inline

: ?adjoin ( elt set -- elt/f )
    2dup in? [ 2drop f ] [ dupd adjoin ] if ;

: (ant) ( total seen stack -- total' )
    [ dup pop pick ?adjoin [ ?walk ] when* ] until-empty drop ;

: ant-no-locals ( -- total )
    0 HS{ } clone V{ } clone 1000 1000 <point> over push (ant) ;