First thing to understand is that while the 9500 is a phone, it is also a computer running a very elegant and powerful operating system. The legacy of the Psion is the availability of a vibrant freeware, shareware, and commercial software industry supporting the device.
The 9500 is the result of developments dating back to the late 1980's when Psion launched the revolutionary Series 3 PDA. The evolution to today's smartphone was not straightforward or without incident.
A good overview is available in this article from The Register., and an analysis of the various OS and UI permutations is here.
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