def add_trees(t1, t2):
    '\n    >>> numbers = tree(1,\n    ...                [tree(2,\n    ...                      [tree(3),\n    ...                       tree(4)]),\n    ...                 tree(5,\n    ...                      [tree(6,\n    ...                            [tree(7)]),\n    ...                       tree(8)])])\n    >>> print_tree(add_trees(numbers, numbers))\n    2\n      4\n        6\n        8\n      10\n        12\n          14\n        16\n    >>> print_tree(add_trees(tree(2), tree(3, [tree(4), tree(5)])))\n    5\n      4\n      5\n    >>> print_tree(add_trees(tree(2, [tree(3)]), tree(2, [tree(3), tree(4)])))\n    4\n      6\n      4\n    >>> print_tree(add_trees(tree(2, [tree(3, [tree(4), tree(5)])]),     tree(2, [tree(3, [tree(4)]), tree(5)])))\n    4\n      6\n        8\n        5\n      5\n    '
    '*** YOUR CODE HERE ***'
>>> numbers = tree(1,
...                [tree(2,
...                      [tree(3),
...                       tree(4)]),
...                 tree(5,
...                      [tree(6,
...                            [tree(7)]),
...                       tree(8)])])
>>> print_tree(add_trees(numbers, numbers))
2
  4
    6
    8
  10
    12
      14
    16
>>> print_tree(add_trees(tree(2), tree(3, [tree(4), tree(5)])))
5
  4
  5
>>> print_tree(add_trees(tree(2, [tree(3)]), tree(2, [tree(3), tree(4)])))
4
  6
  4
>>> print_tree(add_trees(tree(2, [tree(3, [tree(4), tree(5)])]),     tree(2, [tree(3, [tree(4)]), tree(5)])))
4
  6
    8
    5
  5
def add_trees(t1, t2):
    '\n    >>> numbers = tree(1,\n    ...                [tree(2,\n    ...                      [tree(3),\n    ...                       tree(4)]),\n    ...                 tree(5,\n    ...                      [tree(6,\n    ...                            [tree(7)]),\n    ...                       tree(8)])])\n    >>> print_tree(add_trees(numbers, numbers))\n    2\n      4\n        6\n        8\n      10\n        12\n          14\n        16\n    >>> print_tree(add_trees(tree(2), tree(3, [tree(4), tree(5)])))\n    5\n      4\n      5\n    >>> print_tree(add_trees(tree(2, [tree(3)]), tree(2, [tree(3), tree(4)])))\n    4\n      6\n      4\n    >>> print_tree(add_trees(tree(2, [tree(3, [tree(4), tree(5)])]),     tree(2, [tree(3, [tree(4)]), tree(5)])))\n    4\n      6\n        8\n        5\n      5\n    '
    '*** YOUR CODE HERE ***'