// 🔸 rect.cell(i, j)
cell(i, j){
let row = this.row + i - 1;
let col = this.col + j - 1;
if (row < 0) throw new Error(`⛔ RangeRect.cell(): new rect can't start from row = ${row}`);
if (col < 0) throw new Error(`⛔ RangeRect.cell(): new rect can't start from col = ${col}`);
return new RangeRect(row, col, 1, 1);
}