       SippPrism 2dpointlist length shaderhandle [texture]
              Creates   a   prism   defined  by  the  polygon  in
              2dpointlist.  The surface is shaded with the  algo-
              rithm  and  surface parameters specified by shader-
              handle.  2dpointlist is a list of lists of x and  y
              pairs.   The  prism will ascend and descend equally
              far (length / 2.0) from the x-y plane along  the  z
              axis.

              Texture is the texture mapping algorithm to use for
              the object.  The NATURAL texture mapping is similar
              to  CYLINDRICAL  but  the x coordinate is not taken
              from projection on a cylinder but  is  evenly  dis-
              tributed  around the perimeter. An odd thing in all
              the 2D mappings (all except WORLD) for the prism is
              that  the  top  face  will have texture coordinates
              (0.0, 1.0) while the bottom will get (0.0, 0.0).

              See the help file tsipp/primitives/Texture-Map  for
              more details.

              An  object handle that references the new object is
              returned.
