<Feature Face Ring> and <Regular Feature Face>
A <Feature Face Ring> is used to specify a boundary of a <Feature Face>, and is specified as a sequence of <Feature Edge> instances laid end to end.
All <Feature Face> instances may have "holes" in them, represented by <Internal Feature Face Ring>s.
A <Regular Feature Face> instance is required to have an <External Feature Face Ring> as its external boundary.
Detail, Class Diagram Sheet 12