Talking about the difference between the memory access model and the array API, maybe I am talking bullshit (I know next to nothing in these problems) but couldn' an efficient tree data structure be implemented on the memory buffer object ? I am pretty sure a simple tree read-only could, as for a tree that is edited, I am not so sure. Anyhow read-only tree are used a lot by some people. A lab next to mine uses them to describe results from there experiment. They store events in tree-like structures (I have been told that copied that from the CERN). They can then walk throught the tree in a very efficient way, and do statistical analysis on their collection of event. I am not sure if this can fit anywhere in the PEP, but it would sure enlarge its scope. Please enlight me. Gaƫl