it is guaranteed that 
and that if any of
the two is 
so is the other.
interval<T> normalized() const; interval<T>& normalize();
These functions normalize the interval, that is ensure that for an interval it is guaranteed that and that if any of
the two is so is the other.
Returns a copy of the current interval object but in a normalized state.
Normalizes the current interval object and returns a reference to it.