human
Complete this python program:
def _find_common_roots(paths):
assistant
#Here is the Python code you requested.

    """Out of some paths it finds the common roots that need monitoring.
    """
    paths = [x.split(os.path.sep) for x in paths]
    root = {}
    for chunks in sorted(paths, key=len, reverse=True):
        node = root
        for chunk in chunks:
            node = node.setdefault(chunk, {})
        node.clear()
    rv = set()
    def _walk(node, path):
        for (prefix, child) in iteritems(node):
            _walk(child, (path + (prefix,)))
        if (not node):
            rv.add('/'.join(path))
    _walk(root, ())
    return rv