The real proof that reader and writer are correct is that writing then reading gives back exactly what you started with. Today you assert that round-trip on deliberately messy records, the property that validates the whole library.
Show that parsing the writer's output reproduces the original records exactly.
This is the lesson the whole writer was built for. A reader and a writer are only trustworthy together if they are exact inverses, and the way to state that is a round trip: take some records, write them to text, parse that text, and demand the result equals what you started with. If it does, every quoting and escaping decision on both sides agrees, and you can move data through the library without fear of corruption. If it does not, the mismatch points straight at the disagreeing rule.
Choose records that push on every branch at once, a field with the delimiter, a field with a doubled quote, a field with an embedded newline, an empty field, and a field with a significant trailing space, because a round trip is only as strong as the cases it exercises. This is a payoff lesson: if the earlier work is right, the property simply holds and the test needs almost no new code beyond the assertion, which is exactly the satisfying signal you want. Note that round-trip equality holds under the default dialect; a lossy option like trimming deliberately does not round-trip, which is why trimming is opt-in and off by default.
// the property: Parse(Write(records)) deep-equals records// pick records that exercise every writer branch: a delimiter, a quote,// an embedded newline, an empty field, and a significant trailing space// an empty field writes bare and reads back as ""; a trailing space is preserved