L17A
       1. Definition: Orientable Surface
       2. Example: Surface Covered by One Parametrization
 
L17B
       1. Example: Surface Covered by Two Parametrizations
       2. Proposition: A Surface Is Orientable if and only if
           It Has a Differentiable Unit Normal Vector Field
 
L17C
       1. Proposition: A Surface Is Orientable if and only if
           It Has a Differentiable Unit Normal Vector Field (cont.)
       2. Example: Mobius Band