private static void _Backward <A, A_, B, B_>(ITuple <A, A_, B, B_> thiz, int item, object delta, Backpropagation bp)
where A : class, IExpr <A_>
where B : class, IExpr <B_>
{
if (item == 1)
{
thiz.Backward1((A)delta, bp);
}
else if (item == 2)
{
thiz.Backward2((B)delta, bp);
}
else
{
throw new ArgumentException(string.Format("There is no item {0} in this tuple-2.", item));
}
}