[ this ]                  is nil         (   --> [ )

  [ ' [ 1
        [ 2
          [ 4
            [ 7 nil nil ]
            nil ]
          [ 5 nil nil ] ]
        [ 3
          [ 6
            [ 8 nil nil ]
            [ 9 nil nil ] ]
          nil ] ] ]         is tree        (   --> [ )

  [ dup nil = iff drop done
    unpack swap rot
    echo sp
    recurse
    recurse ]               is pre-order   ( [ -->   )

  [ dup nil = iff drop done
    unpack unrot
    recurse
    echo sp
    recurse ]               is in-order    ( [ -->   )

  [ dup nil = iff drop done
    unpack swap
    recurse
    recurse
    echo sp ]               is post-order  ( [ -->   )

  [ queue swap push
    [ dup empty?
        iff drop done
      pop
      dup nil = iff
        drop again
      unpack
      rot echo sp
      dip push push
      again ] ]             is level-order ( [ -->   )

  tree pre-order   cr
  tree in-order    cr
  tree post-order  cr
  tree level-order cr