Was an extra test for empty vectors added at a later time? I see submissions that don't test for that. I had to test for that before my code passed all tests.

I recommend removing step 3 from the instructions. It is very confusing, and not only because of the bad English. The fact that it has its own number and associated text seems to imply that something should be done here, but nothing actually needs to be done in this step. I wasted several minutes trying to decipher this part.

added

added

resolved the ambiguity by specifying that the input strings will only comprise letters

fixed

fixed

Fixed (you need to reset the trainer to see the fix).

The F# version has a bug in the test that refers to the function as "friend", but the boilerplate code uses the name "friendOrFoe". I had to modify the test before I was allowed to submit.

too easy ezezezezezezezezezezezzezezezezezezezezezezezezezezezzezezezezezezezezezezez

Nice idea!

Although it's easy to deduce what you want by looking at the tests, it should be explained more clearly in the instructions what is supposed to be done with the input to create the output.

I initially just did the textbook hello world, but something made me come back and do it in a more interesting way.