The copy and slide instructions are an extension implemented in Whitespace 0.3 and are designed to facilitate the implementation of recursive functions. The idea is that local variables are referred to using [Space][Tab][Space], then on return, you can push the return value onto the top of the stack and use [Space][Tab][LF] to discard the local variables.

added

Thank you for writing this down, I would never have got that double the nth value command right.
Also, that command and the clear n values from the stack command are not part of the whitespace language definition, so how the hell have they ended up here?

If the passed array has no missed letters, your solution will end with ArrayIndexOutOfBoundsException.
For instance, ['a', 'b', 'c', 'd', 'e'] -> ' '.

Junit warning says that it expects java.lang.Exception, not a RuntimeException.

Did you read Nector12's post above?

How are we supposed to do it then ? I think I have errors too.

Re-ranked to 7 kyu

Thanks for this comment! Helped me to understand why I was failing with some test cases.

While I was debugging my solution this kata really felt like 1 kyu. However, if it had example tests fixed and improved (extended) description, it would definitely be worth 2 kyu.

Is the toString() method necessary?

This is strange, because there are 232 different solutions in Go. Please check your solution again.

It's still a problem for Go, points are all correct but due to ordering all tests are failing.