The following paraboloid renders correctly:

   intersection
   { quadric { <1, 0, 1>, <0, 0, 0>, <0, 1, 0>, -1 }
     cylinder { 0, y, 1 }
   }

However, when I extend the clipping cylinder downward:

   intersection
   { quadric { <1, 0, 1>, <0, 0, 0>, <0, 1, 0>, -1 }
     cylinder { -y, y, 1 }
   }

the object disappears completely in POV-Ray 3.7 and 3.7.1.  In POV-Ray 
3.6.1, it renders as expected.

POV-Ray 3.7.0.unofficial
POV-Ray 3.7.1-alpha.8150025.unofficial
openSUSE 13.2 GNU/Linux

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