Well, can some one explain to me, why in the following scene file, one and
only one sphere is appears when it is rendered?  As far as I can tell, the
functions, in effect, should be identical.  Another oddity I have noticed,
is that the max_gradient returned by the first (non-appearing) sphere is
0.0.  And the second, visible sphere does not return a max_gradient value
(and does not when rendered separately).  Why should a declared function in
an isosurface react differently from an explicitly stated one?  Any help or
information would be appreciated, (and I apologize for not posing my
questions better in my previous posts).

The truly puzzled, but still loving isosurfaces, Quadhall

#version unofficial MegaPOV 0.5;
light_source { <0,3,-8> 1 shadowless}
camera { location <0,0,-4> look_at <0,0,0> }
#declare a=function{(x*x)+(y*y)+(z*z)}
isosurface
{
 function {a(x,y,z)}
 contained_by{sphere {0,4}}
 eval
 max_gradient 1
 threshold 1
 pigment{color rgb 1}
 translate <-1,0,0>
}
isosurface
{
 function {(x*x)+(y*y)+(z*z)}
 contained_by{sphere {0,4}}
 eval
 max_gradient 1
 threshold 1
 pigment{color rgb 1}
 translate <1,0,0>
}

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