In the lectures we discussed the recursive implementation of a function which computes the disk usage of a file system tree. In this DiskUsage tree, each node should hold the sum of the disk usage of all of its descendant nodes. The size of an empty DiskUsage tree is 0, and a leaf node is also considered a DiskUsage tree. Here is an example: 26 7 6 3 6 Write a recursive function which returns True if a tree is a valid DiskUsage tree and False otherwise