How to create a sub-function that modify a local variable from the outer function?

Questions
5

This isn’t nice in term of reviewing/maintainability, but obviously not blocking. The exact context in which I needed it was in another question I recently asked

let wrapp-section(
  ...
) = {
  let fn = none
  let fenced = ()

  for it in body.children {
    if (is-heading(it) and it.fields().at("depth") < depth) {
      if fn != none {
        // Complete the last fence
        fn(fenced.join())
        fn = none
        fenced = ()
      }
      // some code…
    } else if is-heading(it) and it.fields().at("depth") == depth {
      if fn != none {
        // Complete the last fence
        fn(fenced.join())
        fn = none
        fenced = ()
      }
      // some other code…
    } else if fn != none {
      // ...
    } else {
      // ...
    }
  }

  // ...
}

As you can see, the following block is repeated two times, and I wanted to create what would be a macro/lambda in other languages to factorize it:

if fn != none {
  // Complete the last fence
  fn(fenced.join())
  fn = none
  fenced = ()
}