anchor.diff:  add to the show. This is done so that a fresh instance of the effect
   is used each time the cue is triggered, in case the effect is complex
   and maintains its own state. The effect creation function will be
  -passed a map containing any <<cue-variables,cue-specific variable bindings>>.
  +passed a map containing any <<user-content-cue-variables,cue-specific variable bindings>>.
   
   There are a number of optional keyword parameters which can be used to
   modify the cue that is created. A summary of some features are
  
positive.code: killed at that point, regardless of how much longer it was planning to
  run.
  
  [[#held-flag]]Cues can also be created which run only as long as the
  corresponding controller pad is held down (this is done by passing a
  true value with the `:held` optional keyword argument when creating
  the cue). This is often done for intense effects like strobes.
  
negative.code: 
  |`:held`
  
  |As described <<user-content-held-flag,above>>, causes the cue's effect to run only
  as long as the corresponding controller button or pad is held down, if
  the controller supports that capability. Currently the web interface
  does not, but the
  