Recall has items.size in the denominator:

- fn = items.size() - tp
- denom = tp + fn
- denom = tp + items.size() - tp = items.size

Bellogin is a fine source for how to define recall.

But I'm now convinced this code is correct.

 I'm not too concerned about supporting ROC curves with this code. We should support them someday, but likely with other code.