NodeIdx leaf(uint32_t i) const { return NodeIdx(i, _leaves); };
NodeIdx parent() { BOOST_ASSERT( not isRoot() ); return NodeIdx(nodeIdx/2, parentlayersize(layerSize)); }
NodeIdx sibling() { return NodeIdx(nodeIdx ^ 0x01, layerSize); }
NodeIdx leaf(uint i) { return NodeIdx(i, _leaves); };