code: func Unwrap(e error) error {
  	if e == nil {
  		return nil
  	}
  	for {
  		w, ok := e.(*merryErr)
  		if !ok {
  			return e
  		}
  		e = w.err
  	}
  }
docs: // Unwrap returns the innermost underlying error.
  // Only useful in advanced cases, like if you need to
  // cast the underlying error to some type to get
  // additional information from it.
  // If e == nil, return nil.