/*
 *  call-seq:
 *     sort_field.to_s -> string
 *
 *  Return a human readable string describing this +sort_field+.
 */
static VALUE
frt_sf_to_s(VALUE self)
{
    GET_SF();
    char *str = sort_field_to_s(sf);
    VALUE rstr = rb_str_new2(str);
    free(str);
    return rstr;
}