LISP PROGRAMMING

Please explain each step with a comment

Write a function future-date which takes four integer parameters: year, month, day and num-days. It will return the future date in num-days days. For example, (future-date 2015 2 3 29) returns (list 2015 3 4) meaning 29 days from Feb 3, 2015 will be Mar 4, 2015. Note that you do need to consider leap years (divisible by 4 but not by 100 unless it's divisible by 400). You may need an auxiliary function to decide the number of days each month has