For review: keeping 234 trees balanced during deletions

[Tyson Richard DOWD]

In cs.mercury you write:

>Zoltan Somogyi <zs at cs.mu.OZ.AU> writes:

>>Tom, please review this.
>>
>>Zoltan.
>>
>>This is the new code of the delete, remove and remove_smallest predicates,
>>and the new predicates they use. This is smaller and clearer than either
>>a unified or a context diff.
>>

>This code looks okay. It remained balanced through a sequence of 10,00,000
>random insertions and deletions.

Perhaps we could add this as a test case (or some variant of it that
runs in reasonable time?).