default long mark() {
    return markState.get();
  }
Retrieves an identifier that represents the current state of the collection.

<p>This marker is tracked globally so getting a mark in one Undoable collection will provide a
mark that can be used in other UndoableCollections

@return a long representing the current state.
evm/src/main/java/org/hyperledger/besu/collections/undo/Undoable.java