I have now opened up derived_quantities and inspected. By searching for n_ret=1, which seems to be what you are looking for, I was able to identify the AngularMomentumVector quantity also has only one return value. In that source code it returns an iterable of length 1. Based on this, I am going to suggest:
Bingo, that worked. Thanks, also, for the reminder to inspect the source for hints :).
Stephen Skory firstname.lastname@example.org http://stephenskory.com/ 510.621.3687 (google voice)