def test_subset_passes_through_all_non_matches():

    """This was used to locate a bug where some of the non-matches had

    gone missing due to mutation of books_trans."""

    statement_trans = [

        S1,  # No match

        S4,  # Match

    ]

    books_trans = [

        B2,  # No match

        B4A, B4B, B4C, # Match

        B3_next_day, B3_next_week,  # No match

    ]

    assert subset_match(statement_trans, books_trans) == (

        [([S4], [B4A, B4B, B4C], [])],  # Matched

        [S1],  # No match: preserved intact

        [B2, B3_next_day, B3_next_week]  # No match: preserved intact

    )