📝 **Task:** Write `binary_search(arr, target)` returning the index of `target` in sorted `arr`, or `-1` if not present. Print `binary_search([1, 3, 5, 7, 9, 11], 7)`. 📋 Implement the function above. Test cases run automatically. 💡 **Hint:** Re-read the theory if you get stuck — the pattern is right there.