    A torus is a 4th order quartic polynomial shape that looks like a donut
 or inner tube.  Because this shape is so useful and quartics are difficult
 to define, POV-Ray lets you take a short-cut and define a torus by:
      torus { MAJOR, MINOR }
 where MAJOR is a float value giving the major radius and MINOR is another
 float specifying the minor radius.  The major radius extends from the center
 of the hole to the mid-line of the rim while the minor radius is the radius
 of the cross-section of the rim.  The torus is centered at the origin and
 lies in the X-Z plane with the Y-axis sticking through the hole.
          ----------- - - - - - - - ----------            +Y
         /          \              /          \            |
        /            \            /            \           |
       |              |          |       |<-B-->|     -X---|---+X
        \            /            \            /           |
         \__________/_ _ _ _ _ _ _ \__________/            |
                           |<----A----->|                 -Y
       A = Major Radius
       B = Minor Radius

    Internally the torus is computed the same as any other quartic or 4th
 order polynomial however a torus defined this way will respond to automatic
 bounding while a quartic must be manually bound if at all.  As with all sha-
 pes, a torus can be translated, rotated and scaled.  Calculations for all
 higher order polynomials must be very accurate.  If this shape renders im-
 properly you may add the keyword 'sturm' after the MINOR value to use POV-
 Ray's slower-yet-more-accurate Sturmian root solver.
