This class is used to handle facets in the system. All facets
in the system have to belong to a facet group. A facet
group is a pike module that inherits from this class. For
example you can create a file MyFacetgroup.pmod with the following
content:
inherit FacetGroup;
You can then use the facet group MyFacetGroup in a facet-class
like this:
class A
{
facet NameOfMyFacet : .MyFacetGroup;
// Rest of class A
}