docs: @Requires("modifier != null")
    @Ensures("getModifiers().contains(modifier)")
    public void addModifier(ElementModifier modifier) {
      switch (modifier) {
        case PRIVATE:
        case PACKAGE_PRIVATE:
        case PROTECTED:
        case PUBLIC:
          modifiers.remove(ElementModifier.PRIVATE);
          modifiers.remove(ElementModifier.PACKAGE_PRIVATE);
          modifiers.remove(ElementModifier.PROTECTED);
          modifiers.remove(ElementModifier.PUBLIC);
      }
      modifiers.add(modifier);
    }