@Override
  public Optional<Bytes> getPriorCode(final Address address) {
    return Optional.ofNullable(code.get(address)).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