def get_hierarchy(ontology, ols_base=None):
    """Iterates over the parent-child relationships in an ontolog

    :param str ontology: The name of the ontology
    :param str ols_base: An optional, custom OLS base url
    :rtype: iter[tuple[str,str]]
    """
    client = OlsClient(ols_base=ols_base)
    return client.iter_hierarchy(ontology)