pub fn interval_min(a: Interval, b: Interval) -> Interval
Interval min: min([a.lo,a.hi], [b.lo,b.hi]) = [min(a.lo,b.lo), min(a.hi,b.hi)].
min([a.lo,a.hi], [b.lo,b.hi]) = [min(a.lo,b.lo), min(a.hi,b.hi)]