I was going to argue that point, but glance at the J docs here:

http://www.jsoftware.com/help/dictionary/d202n.htm

Obverse is defined in a table and some special cases, and there's a built in "this is the obverse of that" instruction.

You could easily tie square/sort together as a pair in Python and it wouldn't be significantly different.

Under(square, sum, mylist) or whatever.