Enumeration Direction

Walk through all parent nodes until reaching the root node

Walk through all children of the parent node of this node while decrementing the index (thus walking left) until you reached the first node.

Walk through all children of the parent node of this node while incrementing the index (thus walking right) until you reached the last node.

Go down until reaching the root and then back up calling the action function on each encountered node

Walk through each children and all grandchildren of the current node until no nodes are left