docs: public List<Sha256Hash> getTransactionHashes() throws VerificationException {
          if (cachedTransactionHashes != null)
              return Collections.unmodifiableList(cachedTransactionHashes);
          List<Sha256Hash> hashesMatched = new LinkedList<>();
          if (header.getMerkleRoot().equals(merkleTree.getTxnHashAndMerkleRoot(hashesMatched))) {
              cachedTransactionHashes = hashesMatched;
              return Collections.unmodifiableList(cachedTransactionHashes);
          } else
              throw new VerificationException("Merkle root of block header does not match merkle root of partial merkle tree.");
      }