code: public final void addHelperText(@NonNull final CharSequence helperText) {
          Condition.INSTANCE.ensureNotNull(helperText, "The helper text may not be null");
          Condition.INSTANCE.ensureNotEmpty(helperText, "The helper text may not be empty");
  
          if (!helperTexts.contains(helperText)) {
              helperTexts.add(helperText);
              verifyPasswordStrength();
          }
      }
docs: Adds a new helper text, which should be shown, depending on the password strength. Helper
  texts, which have been added later than others, are supposed to indicate a higher password
  strength.
  
  @param helperText
  The helper text, which should be added, as an instance of the type {@link
  CharSequence}. The helper text may neither be null, nor empty