docs: function factory(key, options) {
    var settings = options || {}
  
    function one(value) {
      var fn = one.invalid
      var handlers = one.handlers
  
      if (value && own.call(value, key)) {
        fn = own.call(handlers, value[key]) ? handlers[value[key]] : one.unknown
      }
  
      return (fn || noop).apply(this, arguments)
    }
  
    one.handlers = settings.handlers || {}
    one.invalid = settings.invalid
    one.unknown = settings.unknown
  
    return one
  }