Arrowhead

Arrowhead refactored code. 
So, I would much rather see lines 19 & 20 combined, but next time you are inclined to split a statement across two lines, at least tab the trailing lines in so that it's apparent that it's a single statement.

I'm further doubtful about the raise. I don't think it should be here, but rather in `Arrowhead.classify`.

I also don't like the method name. `verify_shape` is one thing that it doesn't do among many that it actually does.