message: [proof] add comments in verify_sparse_merkle_element() for readability
  A variable is renamed as well.
diff: @@ -425,6 +425,7 @@ pub fn verify_sparse_merkle_element(
  (Some(blob), Some((proof_key, proof_value_hash))) => {
  // This is an inclusion proof, so the key and value hash provided in the proof should
  // match element_key and element_value_hash.
  + // `siblings` should prove the route from the leaf node to the root.
  ensure!(
  element_key == proof_key,
  "Keys do not match. Key in proof: {:x}. Expected key: {:x}.",
  @@ -441,8 +442,11 @@ pub fn verify_sparse_merkle_element(
  }
  (Some(_blob), None) => bail!("Expected inclusion proof. Found non-inclusion proof."),
  (None, Some((proof_key, _))) => {
  - // The proof intends to show that proof_key is the only key in a subtree and
  - // element_key would have ended up in the same subtree if it existed in the tree.
  + // This is a non-inclusion proof.
  + // The proof intends to show that if a leaf node representing `element_key` is inserted,
  + // it will break a currently existing leaf node represented by `proof_key` into a
  + // branch.
  + // `siblings` should prove the route from that leaf node to the root.
  ensure!(
  element_key != proof_key,
  "Expected non-inclusion proof, but key exists in proof."
  @@ -453,10 +457,15 @@ pub fn verify_sparse_merkle_element(
  the only existing key, if it existed. So this is not a valid non-inclusion proof."
  );
  }
  - (None, None) => (),
  + (None, None) => {
  + // This is a non-inclusion proof.
  + // The proof intends to show that if a leaf node representing `element_key` is inserted,
  + // it will show up at a currently empty position.
  + // `sibling` should prove the route from this empty position to the root.
  + }
  }
  - let leaf_hash = match sparse_merkle_proof.leaf() {
  + let current_hash = match sparse_merkle_proof.leaf() {
  Some((key, value_hash)) => SparseMerkleLeafNode::new(key, value_hash).hash(),
  None => *SPARSE_MERKLE_PLACEHOLDER_HASH,
  };
  @@ -469,7 +478,7 @@ pub fn verify_sparse_merkle_element(
  .rev()
  .skip(HashValue::LENGTH_IN_BITS - siblings.len()),
  )
  - .fold(leaf_hash, |hash, (sibling_hash, bit)| {
  + .fold(current_hash, |hash, (sibling_hash, bit)| {
  if bit {
  SparseMerkleInternalNode::new(*sibling_hash, hash).hash()
  } else {
  