Function scope_filter 

pub fn scope_filter(
    scope_cats: &[CategoryId],
    rows: &[Vec<Option<String>>],
    cats: &[CategoryId],
    cols: &[String],
) -> Option<(Vec<String>, Vec<Vec<String>>)>

For a binding whose measure ranges over cats, read from dimension columns cols (same order), restricted to the touched rows rows (dimension values in scope_cats order, as Scope::from_rows takes them): the columns of the touched categories it has, and per column the touched values, row-aligned, each tuple once and none with a NULL. None when it has none of the touched categories, so every row is in scope.