Function cell_key 

fn cell_key(
    row_cat: &Option<(CategoryId, String)>,
    col_cat: &Option<(CategoryId, String)>,
    rows: &[(ItemId, String)],
    cols: &[(ItemId, String)],
    pages: &[PageDim],
    row: usize,
    col: usize,
) -> CoordKey

The sorted CoordKey for the cell at [row][col] given the axes/pages. Shared by grid fill and cursor->coordinate mapping so they can’t drift.