Why lowercase `y` and uppercase `X`?  Is that how sklearn does it?  If so, it's fine.
Can we document this a bit more?

```
Mapper kernel for building a random forest classifier.  Each kernel instance fetches the entirety of the feature and prediction (X and y) arrays, and invokes sklearn to create a local decision tree.

The criterion, max_depth, min_samples_split, min_samples_leaf, max_features and bootstrap options are passed to the `sklearn.RandomForest` method.
```