docs: public T reserve(final T key, final int size, final PutFlags... op) {
      if (SHOULD_CHECK) {
        requireNonNull(key);
        checkNotClosed();
        txn.checkReady();
        txn.checkWritesAllowed();
      }
      kv.keyIn(key);
      kv.valIn(size);
      final int flags = mask(op) | MDB_RESERVE.getMask();
      checkRc(LIB.mdb_cursor_put(ptrCursor, kv.pointerKey(), kv.pointerVal(),
                                 flags));
      kv.valOut();
      return val();
    }