GM_OrientableSurface consists of a surface and an orientation inherited from GM_OrientablePrimitive. If the orientation is "+", then the GM_OrientableSurface is a GM_Surface. If the orientation is "-", then the GM_OrientableSurface is a reference to a GM_Surface with an upNormal that reverses the direction for this GM_OrientableSurface, the sense of "the top of the surface" (see 6.4.33.2).<br /></p><p>GM_OrientableSurface:<br /></p><p>{Orientation = "+" implies primitive = self};<br /></p><p>{(Orientation = "-" and TransfiniteSet::contains(p : DirectPosition)) <br /></p><p>  implies (primitive.upNormal(p) = - self.upNormal(p))};<br /></p>