Assuming that I have a token list variable like this:

\tl_set:Nn \l_temp_tl { a b c {e f} g {h} i }.

What is the right way to remove the inner braces and convert it to 

{ a b c e f g h i } 

? (spaces don't matter)

(I want to loop through the list, but I need simply chars as

Ulrike Fischer