interval::hull occludes non-member hull() functions

There are functions `hull(a, b)` and also a static member function `interval::hull(T, T)`. I am using the interval class as a base class. Within my type that is derived from `interval`, I find that it is complicated to invoke the non-member versions of the `hull()` function, because the compiler becomes obsessively glued to the static member function `interval::hull(T, T)`. It effectively occludes the others. Conversely, the static member function serves no purpose, because it performs exactly the same operation as its non-static counterpart. It would be convenient to have this static member function removed.