I implemented an inside edge tracing algorithm, and I made a few corner test cases that my early implementations had issues with that did not have matching problems in the passing criteria for this kata. A couple examples:

1. "Dead Ends":
   A shape that includes a space that an edge tracer needs to completely turn around in:
   +---+
   | |
   | +-+
   +-+

Or if you want a "dead end" in all directions:
+-+
| |
+-+

2. Shapes where corners are right next to each other:
   +---+
   | |
   | ++
   | ++
   +-+

It would be cool to see a few of these corner cases put into the acceptance criteria to make sure everyone's alrogithms are handling them all.