"CONTAINSX" - Finding if a value in table 1 has a matching value in table 2

Perhaps the only thing that makes me happier than a new "X" function (I still badly want a CONCATENATEX) is "inventing" a new one (like we've seen with PRODUCTX). The other day I was looking at a Power Pivot model and thinking "gee, it sure would be nice to have a CONTAINSX." Turns out we can "make" our own CONTAINSX using SUMX.