cargo run printf "%f %f\n" nan -nan
-NaN -NaN
printf "%f %f\n" nan -nan
nan -nan

1. GNU coreutils prints lowercase nan (or NAN with %F, uutils does that too)
2. GNU coreutils preserves the nan sign

1 is trivial to fix, 2 might be a trickier.

Probably not a huge deal in real use cases, but I'm trying to look into refactoring number formatting so I should be able to catch this one as well.