Over at pydata/sparse#102, I ran into some issues about not being able to override where. I was thinking that if there was an if ufunc (suggestions for better names welcome) which selected from either x2 or x3 depending on x1, and made where call that, it would make overriding the three-argument version of where much simpler.

The slight problem I see with this is the following: where currently returns an ndarray, even for scalars. Maybe it'd be better if it decayed 0-d combinations to scalars anyway? I believe so.

Over the long term we might have __array_where__ but for the short term, using existing machinery, this will do.