@larsrh, @tixxit, it's actually about the Double and Float instances in algebra (which we don't test!), and a mismatch between the behavior of quot/mod for algebra Rat and spire Rational.
The algebra Rat does the right thing: x % y = 0
This is consistent with the EuclideanRing laws. However, there is another modulo operation, which is defined on ordered rings. On integers, both definitions overlap, but not on fields nor on polynomial rings.
My PR is on the way. I'm actually working out the laws for this "other" modulo operation.