Write a function cumulative_sum that mutates the Tree t, where each node's label becomes the sum of all entries in the subtree rooted at the node.

def cumulative_sum(t): """Mutates t where each node's root becomes the sum of all entries in the corresponding subtree rooted at t. >>> t = Tree(1, [Tree(3, [Tree(5)]), Tree(7)]) >>> cumulative_sum(t) >>> t Tree(16, [Tree(8, [Tree(5)]), Tree(7)]) """ 

"*** YOUR CODE HERE ***"