struct Context<'a> {
    base: Option<&'a mut Context<'a>>,
    v: &'a mut u32,
}

fn ent<'a: 'b, 'b>(context: &'a mut Context<'b>, v: u32) -> &'a mut u32 {
    let mut current_context = context;
    loop {
        if *current_context.v == v {
            break &mut current_context.v;
        }
        if let Some(base_context) = &mut current_context.base {
            current_context = base_context;
        } else {
            panic!("Entity not found.");
        }
    }
}