Problem 3.1: Cumulative Mul (100pts)

Write a function cumulative_mul that mutates the Tree t so that each node's label becomes the product of all labels in the subtree rooted at the node.

def cumulative_mul(t):
    """Mutates t so that each node's label becomes the product of all labels in
    the corresponding subtree rooted at t.

    >>> t = Tree(1, [Tree(3, [Tree(5)]), Tree(7)])
    >>> cumulative_mul(t)
    >>> t
    Tree(105, [Tree(15, [Tree(5)]), Tree(7)])
    """
    "*** YOUR CODE HERE ***"

Hint: The answer is simple and neat. If your answer is too complicated, you might want to review Section 2.3 and compare the difference between the oop-style Tree and the functional-style tree.