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