// pattern for use in texture/pigment/normal/density
// function pattern using a user defined function
  #declare fn_X = function(x,y,z) { z + x*x + y*y }  // parabolid function
  function { fn_X(x, y, z) }
  //function { fn_X(x, y, z) } 0.5  // for use with normal{} (0...1 or more)