I'm trying to reproduce the following error:

error[E0499]: cannot borrow `cli` as mutable more than once at a time
  --> src/tail.rs:79:67
   |
79 |     Ok( match read_last_n_lines(cli.args().first().unwrap(), lns, cli.get_opt("c") == Some(&OptVal::Empty)) {
   |               ----------------- ---                               ^^^ second mutable borrow occurs here
   |               |                 |
   |               |                 first mutable borrow occurs here
   |               first borrow later used by call

So I created a model example as below:

struct AB {
    a: i16,
    b: i16,
    frozen:bool,
}

impl AB {
    fn set_a(&mut self, v:i16) {
        if !self.frozen {
            self.a = v
        }
    }
    
    fn set_b(&mut self, v:i16) {
        if !self.frozen {
            self.b = v
        }
    }
    fn freeze(&mut self) {
        self.frozen = true
    }
    fn get_a(&mut self) -> i16 {
        self.freeze();
        self.a
    }
    fn get_b(&mut self) -> i16 {
        self.freeze();
        self.b
    }
}
fn sum(a1:i16,a2:i16) -> i16 {
    a1 + a2
}
fn main() {
   let mut ab = AB {a:0,b:0,frozen: false};
   ab.set_a(3);
   ab.set_b(6);
   ab.set_a(1);
   let c = ab.get_b();
   ab.set_a(3);
   println!("sum {}", c + sum(ab.get_a(), ab.get_b()))
}

However, the model has compiled and ran fine. How can I modify it to observe the error?

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