Tree ADT

##### Tree ADT #####
def tree(label, branches=[]):
    """Construct a tree with the given label value and a list of branches."""
    if change_abstraction.changed:
        for branch in branches:
            assert is_tree(branch), 'branches must be trees'
        return {'label': label, 'branches': list(branches)}
    else:
        for branch in branches:
            assert is_tree(branch), 'branches must be trees'
        return [label] + list(branches)

def label(tree):
    """Return the label value of a tree."""
    if change_abstraction.changed:
        return tree['label']
    else:
        return tree[0]

def branches(tree):
    """Return the list of branches of the given tree."""
    if change_abstraction.changed:
        return tree['branches']
    else:
        return tree[1:]

def is_tree(tree):
    """Returns True if the given tree is a tree, and False otherwise."""
    if change_abstraction.changed:
        if type(tree) != dict or len(tree) != 2:
            return False
        for branch in branches(tree):
            if not is_tree(branch):
                return False
        return True
    else:
        if type(tree) != list or len(tree) < 1:
            return False
        for branch in branches(tree):
            if not is_tree(branch):
                return False
        return True

def is_leaf(tree):
    """Returns True if the given tree is a leaf. (i.e. It has no branches.)"""
    return not branches(tree)

def print_tree(t, indent=0):
    """Print a representation of this tree in which each node is
    indented by two spaces times its depth from the root.

    >>> print_tree(tree(1))
    1
    >>> print_tree(tree(1, [tree(2)]))
    1
      2
    >>> numbers = tree(1, [tree(2), tree(3, [tree(4), tree(5)]), tree(6, [tree(7)])])
    >>> print_tree(numbers)
    1
      2
      3
        4
        5
      6
        7
    """
    print('  ' * indent + str(label(t)))
    for b in branches(t):
        print_tree(b, indent + 1)

def copy_tree(t):
    """Returns a copy of t. Only for testing purposes.

    >>> t = tree(5)
    >>> copy = copy_tree(t)
    >>> t = tree(6)
    >>> print_tree(copy)
    5
    """
    return tree(label(t), [copy_tree(b) for b in branches(t)])