fn fill_between(
prev_lhs: Option<LineNumber>,
next_lhs: Option<LineNumber>,
prev_rhs: Option<LineNumber>,
next_rhs: Option<LineNumber>,
) -> Vec<(Option<LineNumber>, Option<LineNumber>)>fn fill_between(
prev_lhs: Option<LineNumber>,
next_lhs: Option<LineNumber>,
prev_rhs: Option<LineNumber>,
next_rhs: Option<LineNumber>,
) -> Vec<(Option<LineNumber>, Option<LineNumber>)>