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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_tree_root(tree, key):
 """Find a root in a tree by it's key :param dict tree: the pkg dependency tree obtained by calling `construct_tree` function :param str key: key of the root node to find :returns: a root node if found else None :rtype: mixed """

    result = [p for p in tree.keys() if p.key == key]
    assert len(result) in [0, 1]
    return None if len(result) == 0 else result[0]