: 4wayIf ( flag flag both neither first second )
  heap [ cons &cons dip [ [ cons ] dip ] dip rot ] preserve
  [ do [ -1 = ] bi@  and ] [ 2drop do drop do ] when
  [ do [  0 = ] bi@  and ] [ 2drop do nip do  ] when
  [ do 0 = swap -1 = and ] [ drop nip  do drop do ] when
  [ do -1 = swap 0 = and ] [ drop nip  do nip do  ] when
  drop 2drop ;