Thanks for finding these, I've updated methods within the reference solution to be defined, and added imports for everything used in the test cases. Also updated to use the new framework annotations and blocks, and moved the reference solution inside a 'random tests' block.

Fixed

I tried to rework the explanation to provide a more concrete mathematical definition of is in the key of, which I thought was probably the most confusing, as well as some more examples. Also, it's a pretty simple problem and the theory section probably isn't very helpful but I still thought I would leave it in, if anyone thinks the sections should be reordered or the theory section should be removed though feel free to comment about it.

Fixed, thanks!

Typos in description: "otehrwise" and "rectrangle"