docs: public void addEmail(String newEmail) {
          Assert.hasText(newEmail, "Attempted to add null or empty email string to user.");
  
          if (emails == null) {
              emails = new ArrayList<>(1);
          }
          for (Email email : emails) {
              if (email.value.equals(newEmail)) {
                  throw new IllegalArgumentException("Already contains email " + newEmail);
              }
          }
  
          Email e = new Email();
          e.setValue(newEmail);
          emails.add(e);
      }