private static void setInheritorsModifiers(List<PsiClass> inheritors) {
    for (PsiClass inheritor : inheritors) {
      PsiModifierList modifierList = inheritor.getModifierList();
      assert modifierList != null; // ensured by absence of anonymous classes
      if (modifierList.hasModifierProperty(PsiModifier.SEALED) ||
          modifierList.hasModifierProperty(PsiModifier.NON_SEALED) ||
          modifierList.hasModifierProperty(PsiModifier.FINAL)) {
        continue;
      }
      modifierList.setModifierProperty(PsiModifier.NON_SEALED, true);
    }
  }
private static void setInheritorsModifiers(List<PsiClass> inheritors)