anchor.diff:  (show/set-variable :my-metro (afterglow.rhythm/metronome 50))
   (require 'afterglow.effects.fun)
   (show/add-effect! :color
  -  (afterglow.effects.fun/metronome-effect sample-show
  +  (afterglow.effects.fun/metronome-effect *show*
       (show/all-fixtures) :metronome :my-metro))
   ----
   
  
positive.code: 
  [source,clojure]
  ----
  (afterglow.rhythm/metro-marker (:metronome sample-show))
  ; -> "15.1.4"
  ----
  
  
negative.code: 
  [source,clojure]
  ----
  (afterglow.rhythm/metro-start (:metronome *show*) 1)
  ; -> 1431727642086
  ----
  
  