Write out a function that will sum up all of the labels in a tree (assume that all labels are ints)
def sum_labels(t):
>>> t = Tree(5, [Tree(1, [Tree(2, [Tree(3)])])])
>>> sum_labels(t)
11
    if ______:
        return ______
    return ______

Write out a function that takes in a tree and a value and return true if n is in the tree and false if it is not.
def is_here(t, n):
>>> t = Tree(5, [Tree(2), Tree(1, [Tree(2, [Tree(3)])])])
>>> is_here(t, 3)
True
    if ______:
        return ______
    return ______

Write out a function that will take in a tree and return a new tree that is identical in structure as the given parameter t, but the value of each node is doubled.
def double(t):
>>> t = Tree(5, [Tree(2), Tree(1, [Tree(2, [Tree(3)])])])
>>> double(t)
Tree(10, [Tree(4), Tree(2, [Tree(4, [Tree(6)])])])
>>> t
Tree(5, [Tree(2), Tree(1, [Tree(2, [Tree(3)])])])
    if ______:
        return ______
    branches = ______
    return Tree(______)

Write out a function that will take in a tree and double the value of every node in the tree (and this time, do it mutatively).
def double_mutative(t):
>>> t = Tree(5, [Tree(2), Tree(1, [Tree(2, [Tree(3)])])])
>>> double(t)
Tree(10, [Tree(4), Tree(2, [Tree(4, [Tree(6)])])])
>>> t
Tree(10, [Tree(4), Tree(2, [Tree(4, [Tree(6)])])])
    t.label = ______
    ______
    ______