How to fold a code inside of an folding in RStudio

I want to make my codes look cleaner and I am already using folding. Whatever I do I can't make a subfold like 2.1 it keeps pretending like another new section. But is there any way to do subfold?

for example

1. a
2. b {foldable}
   • b1 {foldable}
3. c

Even if fold b and b1 i can still see b1 because it thinks b1 is a section, not a subsection.

So are there any way to make sub folding