L23_A
        1. Proposition: Theorem of Turning Tangents

        2. The Integral of a Differentiable Function over a
           Bounded Region on an  Oriented Surface

        3. Theorem: Local Version of Gauss-Bonnet Theorem

L23_B

        1. Theorem: Local Version of Gauss-Bonnet Theorem
            (cont.)

L23_C
        1. Theorem: Local Version of Gauss-Bonnet Theorem
            (cont.)

        2. Theorem: Global Gauss-Bonnet Theorem