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> A vector is a valid expression even if it is cyclic. > > It is well-specified how to handle it: a vector is self-evaluating. > > So, I'd say that we have to be able to handle it. Hmm, I'm not sure I agree. I think you're taking the correspondence between Scheme data and expressions farther than R5RS does. The set of valid Scheme expressions is defined in terms of a context-free grammar, without reference to the structures one can construct. The definition is strictly textual, just like that of most other languages. Since there is no string which corresponds to a cyclic structure, I'd say cyclic structures can't be valid expressions. (The Scheme laywers take out their briefcases! The courtroom goes silent! Doug Evans falls asleep!) :)