+ * tree_flatten() removes pairs of redundant split containers, e.g.:
+ * [workspace, horizontal]
+ * [v-split] [child3]
+ * [h-split]
+ * [child1] [child2]
+ * In this example, the v-split and h-split container are redundant.
+ * Such a situation can be created by moving containers in a direction which is
+ * not the orientation of their parent container. i3 needs to create a new
+ * split container then and if you move containers this way multiple times,
+ * redundant chains of split-containers can be the result.