- This code implements a bit vector that is able to compress long runs of zero bits.
- A binary tree is used. The depth of the tree is determined when the bit vector is created.
- Each leaf node of the tree explicitly represents 128 bits as a pair of uint64 slice elements.
- Each non-leaf node of the tree is a pair of uint64 indexes. A nonzero index points to another node.
- A zero index means that all the bits within the subtree are zero.

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