{{
        call_a_or_b_on_tail!(
            (a: compute_len, b: zero),
                the recursive part that skips over all these
                tokens doesn't much care whether we will call a
                or call b: only the terminal rules care.
        )
}}
{{
        call_a_or_b_on_tail!(
            (a: compute_len, b: zero),
                and now, to justify the existence of two paths
                we will also call a: its input should somehow
                be self-referential, so let's make it return
                some ninety one!
        )
}}
{{
        call_a_or_b_on_tail!(
            (a: compute_len, b: zero),
                and now, to justify the existence of two paths
                we will also call a: its input should somehow
                be self-referential, so let's make it return
                some ninety "(\"()"nine!
        )
}}