But shouldn't people know what function is being used when they don't specify?  We can mention somewhere that (library-wide) we make no guarantees on backward compatibility for the choice of default functions.

> If `flow_func` is `None`, then a default function will be selected, which presently, is `preflow_push`.

But I agree with the general sentiment. In the future, we'd like to be able to algorithmically choose the best function to use, perhaps depending on the graph itself, and so, we don't want to be tied down by backwards compatibility.