Procedure: vector-ref vector k
     K must be a valid index of VECTOR.  `Vector-ref' returns the
     contents of element K of VECTOR.

          (vector-ref '#(1 1 2 3 5 8 13 21) 5) ==> 8
          (vector-ref '#(1 1 2 3 5 8 13 21)
              (let ((i (round (* 2 (acos -1)))))
                (if (inexact? i)
                  (inexact->exact i)
                     i))) ==> 13