@Override
  public Optional<UInt256> getPriorStorageByStorageSlotKey(
      final Address address, final StorageSlotKey storageSlotKey) {
    return Optional.ofNullable(storage.get(address))
        .map(i -> i.get(storageSlotKey))
        .map(PathBasedValue::getPrior);
  }
Locks the layer so no new changes can be added;
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/trie/pathbased/common/trielog/TrieLogLayer.java