L22A
       1. Proposition: A Connected Surface with Every Point
           Being Umbilical Point Is a Piece of Plane or Sphere (cont.)

L22B
       1. Proposition: A Connected Surface with Every Point
           Being Umbilical Point Is a Piece of Plane or Sphere (cont.)
       2. Definition: Asymptotic Direction and Asymptotic Curve
       3. Example: Straight Line

L22C
       1. Example: Straight Line (cont.) and Curve with Positive Curvature
       2. Observation: There Is NO Asymptotic Direction at an Elliptic Point
       3. Definition: Dupin Indicatrix