Well, I have indeed spent more time thinking about this than is probably healthy, but here are some nice edge cases to think about:

- https://github.com/JuliaLang/julia/issues/14826

    mod(-eps(), 4.0)

- https://github.com/JuliaLang/julia/issues/17415

    fld(1.0,0.2)

- https://github.com/JuliaLang/julia/issues/3127

    mod(1.0,0.1)

- https://github.com/JuliaLang/julia/issues/3104

    mod(0.95,0.01)

I'm pretty sure there are more issues.

re: mod 2pi, typically it is most accurate to reduce to (-pi,pi) (i.e. division rounding to nearest). Also to get it accurate you need fancy range reduction algorithms, hence julia has rem2pi https://docs.julialang.org/en/stable/base/math/#Base.Math.re...