def __get_first_mismatch(itr1, itr2):
    """
    Gets the index of the first mismatch (using 'is' not '==') between itr1 and itr2. Returns -1 i
    not found. Extra elements from the longer iterator are ignored.
    """
    return next((i for i, (a, b) in enumerate(zip(itr1, itr2)) if a is not b), -1)