ess than pi/2 radians does not imply the surface
               point is visible to the observer. See the description of
               `visible' below.


   visible     is a logical flag indicating whether the surface point
               is visible to the observer. `visible' takes into account
               whether the target surface occults `spoint', regardless
               of the emission angle at `spoint'. `visible' is returned
               with the value SPICETRUE if `spoint' is visible;
        