bitwise_... functions not working in user defined functions

[ingoogni]

The scene below results in the error "Expected 'function identifier', } found instead".
It seems the three bitwise_... functions are not usable in / available to user defined functions
Could it be made so?

POV-Ray Version

3.7.xx, 3.8.xx

Runtime Environment

Win10

Scene

#version 3.7;
global_settings{ assumed_gamma 1.0 }
#default{ finish{ ambient 0.1 diffuse 0.9 }} 

plane{ 
  <0,1,0>, 0 
  texture{ 
    pigment{ 
      function{bitwise_xor(x, y, z)}
    }
  }
}

light_source{< 3000,3000,-3000> rgb 1}

camera {
  angle 75
  location <0.0 , 3.0 ,-3.0>
  right x*image_width/image_height
  look_at  <0.0 , 1.0 , 0.0>
}