L18A
       1. Proposition: Regular Surface Given by the Inverse
           Image of a Regular Value Is Orientable
       2. Definition: Gauss Map
 
L18B 
       1. Definition: Gauss Map (cont.)
       2. Examples: Plane and Hyperbolic Paraboloid

L18C 
       1. Examples: Hyperbolic Paraboloid (cont.) and Unit Sphere