Predicate, returns whether there is an entry in the database with matching
    IVORN prefix.

def ivorn_prefix_present(session, ivorn_prefix):
    """
    Predicate, returns whether there is an entry in the database with matching
    IVORN prefix.
    """
    n_matches = session.query(Voevent.ivorn).filter(
            Voevent.ivorn.like('{}%'.format(ivorn_prefix))).count()
    return bool(n_matches)