<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def build_base_document(dom, return_fragment=True):
 """ Builds a base document with the body as root. :param dom: Parsed lxml tree (Document Object Model). :param bool return_fragment: If True only <div> fragment is returned. Otherwise full HTML document is returned. """

    body_element = dom.find(".//body")

    if body_element is None:
        fragment = fragment_fromstring('<div id="readabilityBody"/>')
        fragment.append(dom)
    else:
        body_element.tag = "div"
        body_element.set("id", "readabilityBody")
        fragment = body_element

    return document_from_fragment(fragment, return_fragment)