Better define behaviour of Clear() on NonEmptyCollectionImpl

[hectorjsmith]

Requirement

• Design how the Clear method should operate on a NonEmptyCollectionImpl class and implement any necessary changes. The current implementation throws an exception.
• Document the method to clearly define the behaviour.

Value Proposition

Better defining the behaviour makes the class easier to use.

Design Ideas

There are a few options:

1. Throw an exception (as it does today) - it does not make sense to clear a non-empty collection
2. Fail silently - Just leave the collection as it is
3. Somehow transform the collection into an empty collection