code: public <S extends Storable> void unregister(Class<S> type, Cursor<S> cursor) {
          mLock.lock();
          try {
              if (mCursors != null) {
                  CursorList<TransactionImpl<Txn>> cursorList = mCursors.get(type);
                  if (cursorList != null) {
                      TransactionImpl<Txn> txnImpl = cursorList.unregister(cursor);
                      if (txnImpl != null) {
                          txnImpl.unregister(cursor);
                      }
                  }
              }
          } finally {
              mLock.unlock();
          }
      }
docs: Unregisters a previously registered cursor. Cursors should unregister
  when closed.