NotImplementedError not best choice for pending?

[trans]

Using NotImplementedError to indicate a pending test might not be the best idea, b/c it precludes tests that actually fail due to a NotImplementedError. As rare as that may be, it is something to take in to account.

[trans]

We can either designate a new exception class just for this purpose, or limit it to NotImplementedError's that respond true to #assertion?.

We were using the #assertion? method to differentiate between tests needing ASAP implementation, and those that can be postponed indefinitely. But perhaps that distinction is not so important as this.