Wasn't it BorisW37 who wrote:
>sorry to bother you again, but is there a way to make a spiral with a
>trapezoid cross section, without rendering a lot of thin trapezoids rotated
>and with a changing height?
>   __
>  /  \
> /    \     - Trapeziod
>/______\

I'm not quite sure what you mean. 
Is this anything like what you're after?

#include "functions.inc"

camera { location  <0, 1, -4> look_at <0, 0.2 0> angle 35}

light_source {<-100,200,-100> colour rgb 1}

isosurface {
 function {f_helix1(x,y,z,1,10*(2.2+y),0.05*(1.2-y),0.4*(1.2-y),1,0,0)}
   max_gradient 1.2
   contained_by{box {-<1,1,1>,<1,1,1>}}
   pigment {rgb .9}
   finish {phong 0.5 phong_size 10}
   no_shadow
}


-- 
Mike Williams
Gentleman of Leisure

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