[procedure] vector-ref vector k
Curiously, the only reference about vector access time in R6RS is that "the average time needed to access a randomly chosen element is typically less for the vector than for the list." So this doesn't seem to discourage an implementation to use non-flat structure such as trees, or use sparse vectors. Which itself is OK, but seems strange that R6RS explicitly mentions a constant-time access in string-ref.
See also vector-set!.
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