def diff_stitch(input_dsl,output_dsl):
    """
    diffs two stitch dsls and returns their difference.
    Assumes (and asserts) that difference comes at the END of the dsls
    """
    difference = len(output_dsl) - len(input_dsl)
    assert difference >= 0
    if difference != 0:
        assert input_dsl == output_dsl[:-difference], f"input:\n{input_dsl}\noutput:\n{output_dsl}"
        return output_dsl[-difference:]
    else:
        assert input_dsl == output_dsl
        return []