More playing around as I work on documentation. In the attached image 
looking at slope_map{} in the yuqk fork. I believe all of the results 
can be done accomplished in POV-Ray v3.8. too.

Using a scaled onion pattern with an ip_form of <1,0,1> and a 
triangle_wave modifier - POV-Ray's wood without the need to rotate.

In the upper left duplicating a scene from the POV-Ray v3.8 documentation.

In the lower left trying the yuqk way to use the general value modifiers 
within the normal block while also immediately chaining poly_wave.

...
      #declare Fn = function {
         pattern {  // This {} enables value modifiers
             onion
             ip_form <1,0,1>
             triangle_wave
             poly_wave 3
         }
     }

     cylinder {
         0, y, 4
         pigment { srgb <1, .9, .2> }
         finish { specular 1 }
         normal {
     #if (1)
             pigment_pattern { // This {} enables value modifiers
                 onion
                 ip_form <1,0,1>
                 triangle_wave
                 poly_wave 3
             }
     #else
             function { Fn(x,y,z) }
     #end
             slope_map { SlopeMap00 }
           //slope_map { SlopeMap01 }
             bump_size 1.0
             accuracy 0.505 // Larger values are interesting...
         }
     }
...

In the right column changing to larger accuracy values from that in the 
left column produces interesting results. A result of the pyramid of 
four locations, value field, sampling method. Looks cool. :-)

Bill P.

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