anchor.diff:  is being used, and it’s been a long time, since that can drift, unlike
   Pro DJ Link sync:
   
  -code,clojure---------------------------------------------------------
  -code,clojure
  +[source,clojure]
  +----
   (afterglow.rhythm/metro-start (:metronome sample-show) 1)
   ; -> 1431727642086
  ----------------------------------------------------------
  -
  -______________________________________________
  +----
  +
  +____
   The new metronome start timestamp is returned.
  -______________________________________________
  +____
   
   If the beats are landing in the right place but the notion of the down
   beat (bar start) or phrase start is wrong, you can reset those by
  
positive.code: drive a more visceral understanding. To run it for the main show
  metronome, you can do this:
  
  code,clojure--------------------------------------------------
  code,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:
  
  code,clojure-------------------------------------------------------------
  code,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...
  __________________________________
  
  code,clojure--------------------------------------------- code,clojure
  (metro-bpm (show/get-variable :my-metro) 400)
  ---------------------------------------------
  
  __________________________________________
  ...and now they are almost seizure-worthy!
  __________________________________________
  
negative.code: 
  (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:
  
  [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
  