* (both are never defined at the same time)
* returns null if there is no minimum
**/
Facet getMin()
{
Facet minInclusiveFacet= getFacet(Facet.MIN_INCLUSIVE);
if (minInclusiveFacet != null) return minInclusiveFacet;
Facet minExclusiveFacet= getFacet(Facet.MIN_EXCLUSIVE);
if (minExclusiveFacet != null) return minExclusiveFacet;
return null;
}