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immutable structures

Immutable cons structures bother me.

Every other immutable structure in Scheme has a distinct reader
syntax.  (I think numbers and symbols are the only ones although
strings are occasionally candidates.  BTW - I think that would be a
mistake.  Anyone have a use for immutable strings that can't be
satisfied by symbols?)  Even if the necessary immutable-cons?
predicate is added to RRRS, that lack is fatal.

I'm reasonably convinced that there is a fundamental difference
between cons structures and numbers.  Would someone please enlighten
me and justify immutable quoted structures without resorting to
"[some] formal system is cleaner without distinguishing them" as the
best reason?  (Since Scheme procedures have state, mutable quoted
structures are merely syntactic sugar.)