Pre/postorder selection for depth-first search.
These tags are given to goNext and trigger post-order or pre-order traversal of a suffix tree. In case of
PreorderEmptyEdges and PostorderEmptyEdges, the empty edges are also traversed.
Visit the node before its children.
Visit the node after its children.
Visit the node before its children, visit empty edges.
Visit the node after its children, visit empty edges.
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