I've been working with one pecular algebraic data structure, named Register, which is described in currently upgraded http://www.numeric-quest.com/haskell/QuantumComputer.html. or in gzipped version of the same document http://www.numeric-quest.com/haskell/QuantumComputer.html.gz. Section 13 of that document outlines the background for the topic of this message. But the section is just way too long to quote it in here. But to summarize it: data Register is pecular because it is indexable but not observable in a standard way, and because two different representations can describe the same state. In theory there should be well defined transformation from one representation to another. This seems to me as a good subject for some research work. Granted that there are many experts on functional data structures out there (I do not want to pressure any of you gurus, so I am not naming anyone :-)), could you please look at the write-up and help me with the following questions? + Is the Register data structure strangely unique, or does it fit somewhere into a hierarchy of known functional data structures? I would be happy to learn that the latter is the case, since I could then start looking at it at a more formal, well known and tested way. + Is a non-uniqness of representation amenable to formal treatment, such as deforestation? Jan