NonExistenceProof takes a proof of two neighbors, one left of the desired key, one right of the desired key. If both proofs are valid AND they are neighbors, then there is no valid proof for the given key.
Optional
TODO: remove this as unnecessary??? we prove a range
NonExistenceProof takes a proof of two neighbors, one left of the desired key, one right of the desired key. If both proofs are valid AND they are neighbors, then there is no valid proof for the given key.