>You have already got floating point inaccuracy on the input,

Not if they come in as integers that have to be coerced at some point.

>What is the library supposed to do?

Compare ad to bc instead of ad - bc to zero, for one.

>This is a great example of the fallacy of believing that a library automatically solves arbitrary issues.

I didn’t say that and don’t believe it. My point only depends on he library having avoided more domain-specific rookie mistakes than I would have caught in five minutes, when it comes to computational linear algebra. Like the sibling commenter, I don’t think libraries are a panacea, and the answer depends on the specifics of the case.