You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: illegal-usage.rs
    path of file: ./repos/prusti-dev/prusti-tests/tests/verify/fail/type-model
    
    the code of the file until where you have to start completion: use prusti_contracts::*;

use std::vec::Vec;

#[model]
struct Vec<#[concrete] i32> {
    elems: usize,
}

#[extern_spec]
impl Vec<i32> {

    #[ensures( result.model().elems == 0 )]
    fn new() -> Vec<i32>;

    #[ensures( self.model().elems == 1 + old(self.model().elems) )]
    fn push(&mut self, val: i32);
}

#[ensures( result.model().elems == 0 )]
fn create_vec() -
    
fn main() {
    let mut v = create_vec();
    add_elems(&mut v);
    assert!(v.model().elems == 2); //~ ERROR: [Prusti: invalid specification] using models in non-specification code is not allowed
    v.model().elems += 1; //~ ERROR: [Prusti: invalid specification] using models in non-specification code is not allowed


    // Nested usage in closure
    let cl = || {
        assert!(v.model().elems == 2); //~ ERROR: [Prusti: invalid specification] using models in non-specification code is not allowed
    };

    fn nested() {
        let mut v = create_vec();
        assert!(v.model().elems == 2); //~ ERROR: [Prusti: invalid specification] using models in non-specification code is not allowed
    }
}