fn display_line_nums(
    lhs_line_num: Option<LineNumber>,
    rhs_line_num: Option<LineNumber>,
    source_dims: &SourceDimensions,
    display_options: &DisplayOptions,
    lhs_has_novel: bool,
    rhs_has_novel: bool,
    prev_lhs_line_num: Option<LineNumber>,
    prev_rhs_line_num: Option<LineNumber>
) -> (String, String)