macro_rules! recurrence {
    ( a[n] = $($inits:expr),+ , ... , $recur:expr ) => {
        /* ... */
    };
}

fn main() {
    let fib = recurrence![a[n] = 0, 1, ..., a[n-1] + a[n-2]];
    for (i, e) in fib.enumerate().take(10) {
        println!("{} {}", i, e)
    }
}
/*
error: local ambiguity: multiple parsing options: built-in NTs expr ('inits') or 1 other option.
 --> src/main.rs:8:40
  |
8 |     let fib = recurrence![a[n] = 0, 1, ..., a[n-1] + a[n-2]];
  |                                        ^^^
*/