def _serialize_reference(header, obj, indent=12):
    """Serialize REFERENCE.

    Parameters
    ----------
    obj : list
    """
    padding = '  '
    sort_order = {'REFERENCE': 0, 'AUTHORS': 1, 'TITLE': 2, 'JOURNAL': 3,
        'PUBMED': 4}
    for obj_i in obj:
        ref_i = []
        for h in sorted(obj_i, key=lambda k: sort_order.get(k, 100)):
            if h == header:
                s = '{h:<{indent}}{ref}'.format(h=h, indent=indent, ref=
                    obj_i[h])
            else:
                s = '{h:<{indent}}{value}'.format(h=padding + h, indent=
                    indent, value=obj_i[h])
            ref_i.append(s)
        yield '%s\n' % '\n'.join(ref_i)
