Standard output from `dhtmlparser` is single-linked tree. This will make it
    double-linked.

    Args:
        dom (obj): :class:`.HTMLElement` instance.
        parent (obj, default None): Don't use this, it is used in recursive
               call.

def makeDoubleLinked(dom, parent=None):
    """
    Standard output from `dhtmlparser` is single-linked tree. This will make it
    double-linked.

    Args:
        dom (obj): :class:`.HTMLElement` instance.
        parent (obj, default None): Don't use this, it is used in recursive
               call.
    """
    dom.parent = parent

    for child in dom.childs:
        child.parent = dom
        makeDoubleLinked(child, dom)