Add ``<mods:placeTerm>`` tag with proper content.

def add_marccountry_tag(dom):
    """
    Add ``<mods:placeTerm>`` tag with proper content.
    """
    marccountry = dom.find("mods:placeTerm", {"authority": "marccountry"})

    # don't add again if already defined
    if marccountry:
        return

    marccountry_tag = dhtmlparser.HTMLElement(
        "mods:place",
        [
            dhtmlparser.HTMLElement(
                "mods:placeTerm",
                {"type": "code", "authority": "marccountry"},
                [dhtmlparser.HTMLElement("xr-")]
            )
        ]
    )
    insert_tag(
        marccountry_tag,
        dom.match("mods:mods", "mods:originInfo", "mods:place"),
        first(dom.find("mods:originInfo"))
    )