anchor.diff:  
   [source,clojure]
   ----
  -code,clojure
   (afterglow.effects.params/build-variable-param :key
     :type Number :default 0 :adjust-fn #(* % 2))
   ----
  
positive.code: Color parameters are an extremely flexible way of dynamically assigning
  color. The basic way to create one is to call:
  
  code,clojure-------------------------------------------- code,clojure
  (afterglow.effects.params/build-color-param)
  --------------------------------------------
  
  By itself this call would simply return a non-dynamic back color.
  However, you will use one or more of the following optional keyword
  
negative.code: dynamic parameter. Afterglow tries to be as efficient about this as
  possible, and do as much calculation as it can when the parameter is
  created. If there are no frame dynamic parameters, it will return a
  fixed color. But you can easily use frame-dynamic oscillated
  parameters and get lovely shifting rainbow cues, as shown in the
  <<effect_functions#oscillator-effects,effect examples>>.
  
  == Direction Parameters
  
  Direction parameters are a way to tell a group of fixtures to point in a
  particular direction, or move in unison or in a coordinated pattern. The
  