Class TopDownTreeIterator

Let assume the following tree:

  R
  |
  +---1
  |   |
  |   +---1.1
  |   |
  |   +---1.2
  |   |
  |   +---1.3
  |   |    |
  |   |    +---1.3.1
  |   |    |
  |   |    +---1.3.2
  |   |
  |   +---1.4
  |
  +---2
      |
      +---2.1

When selecting 1.2 as the root, the normal DepthFirstTreeIterator would stop on 1.2 as it does not have children, but this iterator will visit the next sibling (1.3 and 1.4 but not 1.1) nodes. So the expected traversal order will be 1.2, 1.3, 1.3.1, 1.3.2, and 1.4 then jumps to 2 and continues with 2.1.