code: function off (state, eventName, handler) {
    state.emitter.removeListener(eventName, handler)
  
    return state.api
  }
docs: removes a listener for the specified event
  
  It will unsubscribe at most, one instance of a listener for a particular event.
  If any single listener has subcribed multiple times to the same event,
  then `off` must be called multiple times.
  
  Supported events:
  
  - `add`
  - `update`
  - `remove`
  - `change`
  
  @param  {String} eventName   Name of event, one of listed above
  @param  {Function} handler   callback for event