Book Index Contents with subcategories

Hello guys,

Would be much appreciated if the book have on its index/Contents the sub categories described in the Contents summary. For example: In the book Data Structures and Algorithms in Swift, you would describe: Chapter 2: Complexity, 2.1 - Time Complexity, 2.1.1 - Constant Time, 2.1.1 - Linear Time …

It would be by far mor easy to find thing we already learned and want to consult after.

Thanks in advance.

@ygornascimento Thank you for your suggestion - much appreciated! I will forward it to the book team.