}

    /**
     * Spock helper to mock Guava's immutable collections and maps with empty instances.
     */
    static class GuavaImmutablesResponse implements IDefaultResponse {
        static final IDefaultResponse INSTANCE = new GuavaImmutablesResponse()

        @Override
        Object respond(IMockInvocation invocation) {
            if (ImmutableCollection.isAssignableFrom(invocation.method.returnType)