int getFirstUnmarkedItem() {
    for (int i = 0; i < marked.length; i++) {
      if (!marked[i]) {
        return i;
      }
    }
    return -1;
  }
Walks the taken list and returns the first unmarked item

@return the first unmarked item, or -1 if all items are marked.
evm/src/main/java/org/hyperledger/besu/evm/code/WorkList.java