L7_A
      1. Line of Striction

L7_B
      1. Condition of a Ruled Surface Given by the Line of
          Striction as Directrix being a Regular Surfac
      2. Gauss Curvature of a Ruled Surface Given by the Line
          of Striction as Directrix

L7_C
      1. Developable Surface
      2. Developable Surface Has Gauss Curvature Zero at
          Regular Points