Tree traversal

{wiki}

The summary from https://www.geeksforgeeks.org/tree-traversals-inorder-preorder-and-postorder/ is a winner:
``
    1
   / \
  2   3
 / \
4   5
``
* <inorder DFS>: 4 2 5 1 3
* <preorder DFS>: 1 2 4 5 3
* <postorder DFS>: 4 5 2 3 1
* <breadth-first search>: 1 2 3 4 5

In principle one could talk about tree traversal of <unordered trees> as a number of possible traversals without a fixed order. But we won't consider that under this section, only deterministic <ordered tree> traversals.
