Your exercise can't:
1. Tell me who other people's friends are.
2. Change it's output if a spammer stops pinging after 1 hour.
3. Count the amount of times a person has pinged in their lifetimes.
4. Build a pyramid
5. Output Pi
6. Make me lasagna
For some “can't” statements, you don't need to write any error.
The “can't” statements in CDD are for actions and input that you could take, but the software deliberately prevents you from doing them.
There are no actions or input, requested from the exercise that make it possible to do the list you gave me, so it does not make sense to create an error for them.
The TDD and DDD ultimately tell you which inputs are of interest. CDD doesn't, so it's apples to oranges.
People who think that specifically TDD is a programming technique didn't get the entire memo. It can drive you to design and re-design your app's inputs. But that part of TDD is tacit - hard to teach.
