fn push_both_delimiters<'s, 'b>(
    entered: &Stack<'b, EnteredDelimiter<'s, 'b>>,
    lhs_delim: &'s Syntax<'s>,
    rhs_delim: &'s Syntax<'s>,
    alloc: &'b Bump
) -> Stack<'b, EnteredDelimiter<'s, 'b>>