Function sorted_kv_expr_with 

fn sorted_kv_expr_with(
    sorted_cols: &[String],
    nullable: bool,
    col: &dyn Fn(&str) -> Expr,
) -> Expr

Renders pre-sorted columns as a k=v,k=v concatenation. nullable coalesces each value to '' (id columns are tags and non-null; scope columns carry no such guarantee).