> example:
> #declare Norm = <0, 0, 0>;
> #declare Start = <x-pos, 20, z-pos>;
>
> testing for an intersection on a surface perpendicular to y, would be 
> written as:
> #declare Inter=
>  trace ( MySphere, Start, -y, Norm );
>  etc....
>  )
>
> That is how I do it.
>
> Thomas

Also, if you want to test for a missed ray you should test the
normal for zero length.

#if (vlength(Norm)=0)
   //missed
#else
   //hit
#end

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