 PRHS                                                               Menu: PRISM
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 Creates a hollow rectangular prism or two (2D) rectangular nested polygons,
 given a starting point and dimensions.

         1. * (CHAR) plane of base rectangle (x-xy,y-yz,z-zx) <x>
         2. * (POINT) starting point
      for options `x' or `z':
         3. * (NUM) length on x
      for option `y':
         3. * (NUM) length on y
      for option `x':
         4. * (NUM) length on y
      for options `y' or `z':
         4. * (NUM) length on z
      for all options:
         5. * (NUM) "wall" thickness (greater than 0)
      for option `x':
         6. * (NUM) height of prism
      for options "y" or "z":
         6. * (NUM) length of prism

      for all options:
         7. * (Y/N) positive prism ? <y>

 Description:
 This command creates a set of two nested rectangular polygons or a hollow
 rectangular prism based on the polygons. Specify a base plane: one of the
 axial planes; (x  xy plane, the default; y  yz plane, z  xz plane). Define
 a starting point and specify the dimensions along the x and y axes (for `x'
 option), or along the y and z axes (for `y' option), or along the x and z axes
 (for `z' option). Specify the wall thickness and the height (for `x' option)
 or the length (for `y' and `z' options) (you may enter `0' for a 2D polygon).
 Enter `y' (the default) for a positive prism, or `n' for a negative prism. You
 are prompted to create more prisms until another command is selected.

         Example: \prhs x 0 0 0 4 7 0.2 3 y

 Result: A rectangular hollow prism is created with its lower-left corner at
 the point 0 0 0. It is 4 units long in the x direction, 7 in the y direction,
 its "wall" thickness is 0.2 units and its height is 3 units.
