% SPDX-FileCopyrightText: 2021-2023 The Refinery Authors <https://refinery.tools/>
%
% SPDX-License-Identifier: EPL-2.0
problem builtin.

abstract class node.

pred exists(node).

pred equals(left, right).

abstract class contained extends node.

pred contains(container, contained contained).

error invalidContainer(contained contained).