public static Bag <Pair <Option <A>, Option <B> > > Join <K, A, B>(
this Bag <A> a,
Bag <B> b,
Func <A, K> aKey,
Func <B, K> bKey,
Func <A, A, A> aReduce,
Func <B, B, B> bReduce)
{
var ak = a.SelectWithKey(aKey);
var bk = b.SelectWithKey(bKey);
return(ak.Join(bk, aReduce, bReduce).Select(v => v.Value));
}