L23A
        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

L23B
        1. Theorem: Local Version of Gauss-Bonnet Theorem
            (cont.)

L23C
        1. Theorem: Local Version of Gauss-Bonnet Theorem
            (cont.)
        2. Theorem: Global Gauss-Bonnet Theorem