#[derive(Debug, Clone)]
pub enum ChunkContent<'a> {
    Text(&'a str),
    Tokens(Box<Vec<Token>>),
    Block(Box<Branch<'a>>),
}

#[derive(Debug, Clone)]
pub struct Chunk<'a> {
    pub content: ChunkContent<'a>,
}

#[derive(Debug, Clone)]
pub struct Branch<'a> {
    pub content: Vec<Chunk<'a>>,
}

//
// other place in other module
//

pub fn tokenize_branch_mutate(branch: &mut Branch<'a>) {
     for mut chunk in branch.content.iter() {
         match chunk.content
            ChunkContent::Text(t) => {

                // !!!
                // Cannot assign immutable field 'chunk.content'

                chunk.content = ChunkContent::Tokens(Box::new(Self::tokenize(t)));
            },

            ChunkContent::Block(ref mut b) => { // Cannot borrow immutable private field '... Block).0' as mutable
                Self::tokenize_branch_mutate(b);
            },

            _ => continue,
        }
     }
}