Help in fix - cannot borrow data in a `&` reference as mutable?

help
2 
// Change this:
match &opt.v {
    &Some(OptVal::Arr(ref mut set)) => {
        // your code here
    }
    _ => {}
}

// To this:
match &mut opt.v {
    Some(OptVal::Arr(set)) => {
        // your code here
    }
    _ => {}
}

// Or use if-let (cleaner):
if let Some(OptVal::Arr(set)) = &mut opt.v {
    // your code here
}

The problem: you used &opt.v (immutable) but tried to get ref mut set (mutable). You need &mut opt.v to modify the data inside.