@_ is [[3, 1]]
receiving row 3 col 1
    @_ is [[2, 0], [3, 1]]
    receiving row 2 col 0
        @_ is [[1, 0], [2, 0], [3, 1]]
        receiving row 1 col 0
            @_ is [[0, 0], [1, 0], [2, 0], [3, 1]]
            receiving row 0 col 0
            RETURNING: [[0, 0], [1, 0], [2, 0], [3, 1]]
    @_ is [[2, 1], [3, 1]]
    receiving row 2 col 1
        @_ is [[1, 0], [2, 1], [3, 1]]
        receiving row 1 col 0
            @_ is [[0, 0], [1, 0], [2, 1], [3, 1]]
            receiving row 0 col 0
            RETURNING: [[0, 0], [1, 0], [2, 1], [3, 1]]
        @_ is [[1, 1], [2, 1], [3, 1]]
        receiving row 1 col 1
            @_ is [[0, 0], [1, 1], [2, 1], [3, 1]]
            receiving row 0 col 0
            RETURNING: [[0, 0], [1, 1], [2, 1], [3, 1]]