anchor.diff:  
   (afterglow.rhythm/snapshot-beat-within-bar snap)
   ; -> 2
  -----------------------------------------------------------------------------------------------
  +----
   
   Since the notion of a “down beat” (the first beat in a bar) is important
   in music and thus potentially to lighting effects, there is a snapshot
   function for checking that, too:
   
  -code,clojure------------------------------------------- code,clojure
  +[source,clojure]
  +----
   (afterglow.rhythm/snapshot-down-beat? snap)
   ; -> false
  --------------------------------------------
  -
  -_____________________________________
  +----
  +
  +____
   The second beat is not the down beat.
  -_____________________________________
  +____
   
   The metronome can be restarted at a particular beat by calling
   `metro-start` with a beat number. (Calling it with no argument returns
  
positive.code: http://deepsymmetry.org/afterglow/doc/afterglow.rhythm.html[afterglow.rhythm]
  specify and describe them.
  
  [[watching-metronomes]]
  Watching Metronomes
  +++++++++++++++++++
  
  To help get a feeling for exactly what your metronome is doing, there is
  an effect function that flashes one bright color on the down beat, and
  
negative.code: drive a more visceral understanding. To run it for the main show
  metronome, you can do this:
  
  [source,clojure]
  ----
  (require 'afterglow.effects.fun)
  (show/add-effect! :color
    (afterglow.effects.fun/metronome-cue sample-show
      (show/all-fixtures sample-show)))
  ----
  
  If you want to watch a different metronome, the invocation looks like
  this:
  
  [source,clojure]
  ----
  (show/set-variable :my-metro (afterglow.rhythm/metronome 50))
  (require 'afterglow.effects.fun)
  (show/add-effect! :color
    (afterglow.effects.fun/metronome-cue sample-show
      (show/all-fixtures) :metronome :my-metro))
  ----
  
  ____
  The lights are pulsing sedately...
  ____
  
  [source,clojure]
  ----
  (metro-bpm (show/get-variable :my-metro) 400)
  ----
  
  ____
  ...and now they are almost seizure-worthy!
  ____
  