> It all comes down to what is in the interface contract.  Anything you
> document becomes part of that contract to the outside world, and thus
> difficult to change.

That depends.  *Good* interface contract documentation includes things
like "what happens if hashdone is called on a non-empty hash table is
undefined", specifically giving the implementation latitude to change.
While it doesn't actually *prevent* people from testing (or looking) to
see what the implementation does and then depending on it, it *does*
make it clear whose fault it is when an implementation change breaks
the resulting code.

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