error_unk_class.nit:16,9--12: Error: class `Fail` not found in module `error_unk_class`.
error_unk_class.nit:17,9--15: Error: class `Boolean` not found in module `error_unk_class`. Did you mean `core::Bool`?
error_unk_class.nit:18,9--16: Error: class `core::list::ListNode` not visible in module `error_unk_class`.
error_unk_class.nit:19,9--13: Error: class `POSet` not found in module `error_unk_class`. Did you mean `core::Set`?