code: func (v *Context) Enter(title string, args ...interface{}) {
  	e := fmt.Sprintf(title, args...)
  	v.element = append(v.element, e)
  }
docs: // Enter descends into a sub-element when validating a nested structure.
  //
  // Useful for defining context. A current path of elements shows up in
  // validation messages.
  //
  // The reverse is Exit.