(seq
    (seq
        (seq
            (seq
                (seq
                    (ap "a1" $stream_1)
                    (ap "a2" $stream_1)
                )
                (seq
                    (ap "b1" $stream_2)
                    (ap "b2" $stream_2)
                )
            )
            (seq
                (ap "c1" $stream_3)
                (ap "c2" $stream_3)
            )
        )
        (seq
            (ap "d1" $stream_4)
            (ap "d2" $stream_4)
        )
    )
    (par
        (par
            (xor
                (fold $stream_1 v1
                    (seq
                        (fold $stream_2 v2
                            (xor
                                (seq
                                    (seq
                                        (fold $stream_3 v3
                                            (par
                                                (fold $stream_4 v4
                                                    (par
                                                        (call "{fold_executor_id}" ("" "") [])
                                                        (next v4)
                                                    )
                                                )
                                                (next v3)))
                                        (call "{error_trigger_id}" ("error" "") []) ; will trigger an error
                                    )
                                    (next v2)
                                )
                                (ap %last_error% $error_stream)
                            )
                        )
                        (next v1)
                    )
                )
                (call "{last_error_receiver_id}" ("" "") [%last_error%]) ; shouldn't be called
            )
            (seq
                (call "{last_peer_checker_id}" ("" "") ["last_peer 2"])
                (fold $error_stream error
                    (seq
                        (call "{last_peer_checker_id}" ("" "") [error])
                        (next error)
                    )
                )
            )
        )
        (call "{last_peer_checker_id}" ("" "") ["last_peer"])
    )
)