ORIENTED MANIFOLD 2D
Definition
An object is an <ORIENTED MANIFOLD 2D> if and only if: it is a <ORIENTED MANIFOLD 2D>; it has an orientation. The neighbourhood of each point in a <MANIFOLD 2D> is isomorphic to a region of R2. The concept of orientation requires an 'above' and a 'below'. Hence for an <ORIENTED MANIFOLD 2D> the neighbourhood of a each point (including points 'above' and 'below' that are outside the 2D manifold) is isomorphic to a region of R3