Function fixed_point 

pub fn fixed_point<Input, Output>(
    tracing_span: impl Fn(&Input) -> Span,
    storage: &'static LocalKey<RefCell<FixedPointStack<Input, Output>>>,
    args: Input,
    default_value: impl Fn(&Input) -> Output,
    next_value: impl Fn(Input) -> Output,
) -> Output
where
    Input: Value,
    Output: Value,