Implication World level 9 - unable to use `zero_ne_succ` on regular rules

[nvchhawch]

Using it produces the following error message:

You have not unlocked the lemma/definition 'MyNat.zero_ne_succ' yet!

I don't believe it's possible to complete this level without using this axiom. Please correct me if I'm wrong.

[nvchhawch]

I think 75bb4e5 removed zero_ne_succ accidentally.

[joneugster]

Fixed in #32.