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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 double_linked_dom(str_or_dom):
 """ Create double linked DOM from input. In case of string, parse it, make it double-linked. In case of DOM, just make it double-linked. Args: str_or_dom (str/HTMLelement):
 String or HTMLelement instance. Returns: obj: HTMLelement with parsed, double-linked content from `str_or_dom`. """

    dom = str_or_dom
    if not isinstance(str_or_dom, dhtmlparser.HTMLElement):
        dom = dhtmlparser.parseString(str_or_dom)

    dhtmlparser.makeDoubleLinked(dom)

    return dom