def explain_node_str(root, indent=0):
    """Returns an indendeted outline-style representation of the subtree.
    """
    indent_string = "    " * indent

    buf = f"{indent_string}Node<item={root[0]}>"

    if not root[1] and not root[2]:
        buf += "\n"
    else:
        buf += ":\n"

    if root[1]:
        buf += f"{indent_string} -Left:\n"
        buf += explain_node_str(root[1], indent + 1)

    if root[2]:
        buf += f"{indent_string} -Right:\n"
        buf += explain_node_str(root[2], indent + 1)

    return buf