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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _bddnode(root, lo, hi):
 """Return a unique BDD node."""

    if lo is hi:
        node = lo
    else:
        key = (root, lo, hi)
        try:
            node = _NODES[key]
        except KeyError:
            node = _NODES[key] = BDDNode(*key)
    return node