I also have a weight-balanced tree implementation (quite experimental,
but there are tests), in containers.data: `CCWBTree`
(http://cedeela.fr/~simon/software/containers/CCWBTree.S.html).
It should provide all the operations you ask for in O(log(n)), and takes
a total order as a parameter.

Cheers,

Le Wed, 04 Nov 2015, Francois Berenger a écrit :
> I am looking for even a simple implementation with at
> least the following operations:
> 
> insert/add, remove, mem/contains and at_rank.
> 
> The at_rank is especially important since it is inefficient
> to implement it using fold in a set like the ones from the stdlib.
> 
> Thanks a lot,
> F.


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Simon Cruanes

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