Yes obviously Java is going to be the tool at hand, but to keep focus on the good aspects of this exercise (naming, testing, etc) perhaps it would be best to have a problem that has a good solution in Java.

If the subject does this property with generics that simple sum type alone requires writing several classes just to represent the sum type.

It's not clear from the article what the input is. What signature I can use depends on what the input data is. Is it a string rep of the nested list? Can I assume it's any type I want such as my own NestedList?