code: func (fn typeFunc) Describe(exact interface{}, extra ...interface{}) (string, error) {
  	values := []reflect.Value{reflect.ValueOf(exact)}
  	for _, obj := range extra {
  		values = append(values, reflect.ValueOf(obj))
  	}
  	out := fn.Fn.Call(values)
  	s := out[0].Interface().(string)
  	var err error
  	if !out[1].IsNil() {
  		err = out[1].Interface().(error)
  	}
  	return s, err
  }
docs: // Describe invokes the nested function with the exact number of arguments.