ET-trees data structure implementation.
Contributed by Pavel Nejedly
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The ET-forest structure is described in:
D. D. Sleator and R. E. Tarjan. A data structure for dynamic trees.
J. G'omput. System Sci., 26(3):362 381, 1983.
We do not enable this with CHECKING_P, since it is awfully slow.
The occurrence of a node in the et tree.