Ahh. I see now that this targets a particular test case... I'm not sure what the neatest solution is; I don't know any reason an estimator should exist without `predict` but with `decision_function`. And maybe by definition of the documented "prediction API" it must have `predict`. Scikit-learn needs to do some work in general on working out how to identify estimators with particular capabilities.

So perhaps leave it as it is, but in an `elif`.