if a and b are product types (a1&a2 and b1&b2) then

a1 must be bindable to b1,
a2 to b2 and so on..

remember:subtypes may have more than 2 types or may be different, so check for length

I guess that you've modified the input pattern like in mixbonacci.

Actually, the reference solution and the tests were correct. The problem was that your solution mutated the input pattern (which isn't a good practice in general). I've modified the tests to enable this type of solutions.

Regards,

suic