docs: public T reserve(final Txn<T> txn, final T key, final int size,
                     final PutFlags... op) {
      if (SHOULD_CHECK) {
        requireNonNull(txn);
        requireNonNull(key);
        txn.checkReady();
        txn.checkWritesAllowed();
      }
      txn.kv().keyIn(key);
      txn.kv().valIn(size);
      final int flags = mask(op) | MDB_RESERVE.getMask();
      checkRc(LIB.mdb_put(txn.pointer(), ptr, txn.kv().pointerKey(), txn.kv()
                          .pointerVal(), flags));
      txn.kv().valOut(); // marked as in,out in LMDB C docs
      return txn.val();
    }