[OGC99-049] states that "the boundary of a geometry is a set of geometries of the next lower dimension. The boundary of a Point or MultiPoint is the empty set. The boundary of a non-closed Curve consists of itw two end Points, the boundary of a closed Curve is empty. The boundary of a MultiCurve consists of those Points that are in the boundaries of an odd number of its element Curves. The boundary of a Polygon consists of its set of Rings. The boundary of a MultiPolygon consists of the set of Rings of its Polygons. The boundary of an arbitrary collection of geometries whose interiors are disjoint consists of geometries drawn from the boundaries of the element geometries by application of the 'mod 2' union rule."