Develop the function ft-descendants. It consumes a descendant family tree (see section 15.1) based on the following structure definition:
![2449_d10cb745-3be1-4c6e-9166-ab628dd7bc3b.png](https://secure.tutorsglobe.com/CMSImages/2449_d10cb745-3be1-4c6e-9166-ab628dd7bc3b.png)
The last field in a parent structure is originally 0. The function ft-descendants traverses the tree and modifies these slots so that they contain the total number of descendants of the corresponding family member. Its result is the number of total descendants of the given tree.