def get_concept_scheme(rdf):
    """Return a skos:ConceptScheme contained in the model.

    Returns None if no skos:ConceptScheme is present.
    """
    # add explicit type
    for s, o in rdf.subject_objects(SKOS.inScheme):
        if not isinstance(o, Literal):
            rdf.add((o, RDF.type, SKOS.ConceptScheme))
        else:
            logging.warning(
                "Literal value %s for skos:inScheme detected, ignoring.", o)
    css = list(rdf.subjects(RDF.type, SKOS.ConceptScheme))
    if len(css) > 1:
        css.sort()
        cs = css[0]
        logging.warning(
            "Multiple concept schemes found. "
            "Selecting %s as default concept scheme.", cs)
    elif len(css) == 1:
        cs = css[0]
    else:
        cs = None

    return cs