public void accept(@NotNull PsiElementVisitor visitor) {
   if (visitor instanceof BnfVisitor) ((BnfVisitor) visitor).visitExternalExpression(this);
   else super.accept(visitor);
 }
 public void accept(@NotNull PsiElementVisitor visitor) {
   if (visitor instanceof BnfVisitor) ((BnfVisitor) visitor).visitParenthesized(this);
   else super.accept(visitor);
 }