def maybe_append(usa_facts, jhu):
    """
    Append dataframes if available, otherwise return USAFacts.

    If both data frames are available, append them and return.

    If only USAFacts is available, return it.

    If USAFacts is not available, return None.
    """
    if usa_facts is None:
        return None
    if jhu is None:
        return usa_facts
    return usa_facts.append(jhu)