項16.5.2. 「Geometry Functions」, discusses
          several functions that construct new geometries from existing
          ones. See that section for descriptions of these functions:
        
              Envelope(
            g)
              StartPoint(
            ls)
              EndPoint(
            ls)
              PointN(
            ls,N)
              ExteriorRing(
            poly)
              InteriorRingN(
            poly,N)
              GeometryN(
            gc,N)

