/*
 * call-seq:
 *      causal? -> bool
 *
 * +true+ iff the message's service type is Spread::FIFO_MESS.
 */
static VALUE
data_msg_is_causal(VALUE obj)
{
    struct SpreadMessage *sp_mess;

    Data_Get_Struct(obj, struct SpreadMessage, sp_mess);
    return INT2BOOL(Is_causal_mess(sp_mess->service_type));
}