anchor.diff:  http://deepsymmetry.org/afterglow/doc/afterglow.effects.cues.html#var-cue[cue]
   function creates a cue. At its most basic, you pass in two parameters,
   `show-key` which is the keyword that will be used to add the
  -cue&rsquo;s effect to the show when the cue is triggered, ending any
  +cue's effect to the show when the cue is triggered, ending any
   other effect running under that same keyword, and `effect`, which is a
   function that will be called when the cue is triggered, and whose
   responsibility is to create and return the effect that the cue should
  
positive.code: |Sets the color of the cue within the grid for hinting about its
  purpose and relatedness to other cues, to help operators learn and
  understand the interface. If not specified, white is used.
  
  |===
  
  
negative.code: by the cue can be updated by the aftertouch pressure exerted by the
  operator as they hold down the pad. This can make for very expressive
  effects, as exemplified by the Sparkle cue set up early in the
  examples namespace's
  http://deepsymmetry.org/afterglow/doc/afterglow.examples.html#var-make-cues[make-cues]
  function, and its
  http://deepsymmetry.org/afterglow/doc/afterglow.examples.html#var-make-strobe-cue[strobe]
  