Previously, the radix iterator unconditionally found the next non-null leaf node, even if it fell outside of the range the iterator was derived from. This was obviously an efficiency issue, but it also introduced unnecessary sharing because of the reads outside of the iterator's range. Now it takes an explicit upper bound and will stop resolving elements when it reaches that bound. The end() iterator is now naturally represented in terms of this bound, rather than in terms of a special-case key. For asharing vm, this eliminates all violations of the commutativity rule. Since I couldn't figure out how to nicely work this bound into the existing recursive traversal implementation, I reworked it to be iterative and, I think, made it much simpler in the process. This does make one notable semantic change: previously the radix iterator cached the element it pointed to, so an update to that element would not affect the value returned by the iterator. It no longer caches it, which makes it behave more like a regular collection iterator.