Function brujin_map_f_internal 

pub fn brujin_map_f_internal<X: Binder, Y: Binder, M>(
    t: GTerm<X, Infallible>,
    m: &BTreeMap<<X as Binder>::Var, usize>,
    depth: usize,
    into: &mut impl FnMut(usize) -> Y,
) -> GTerm<Y, M>
where
    <X as Binder>::Var: Eq + Ord + Clone,