code: function factory(key, state, ctx) {
    return enter
  
    function enter() {
      var context = ctx || this
      var current = context[key]
  
      context[key] = !state
  
      return exit
  
      function exit() {
        context[key] = current
      }
    }
  }
docs: /* Construct a state `toggler`: a function which inverses
  `property` in context based on its current value.
  The by `toggler` returned function restores that value.