code: func ValidUsernamePassword(username, password string) error {
  	const allowed_letters = "abcdefghijklmnopqrstuvwxyzæøåABCDEFGHIJKLMNOPQRSTUVWXYZÆØÅ_0123456789"
  NEXT:
  	for _, letter := range username {
  		for _, allowedLetter := range allowed_letters {
  			if letter == allowedLetter {
  				continue NEXT // check the next letter in the username
  			}
  		}
  		return ErrInvalidCharacters
  	}
  	if username == password {
  		return ErrUsernameAsPassword
  	}
  	return nil
  }
docs: // Check that the given username and password are different.
  // Also check if the chosen username only contains letters, numbers and/or underscore.
  // Use the "CorrectPassword" function for checking if the password is correct.