(rposition haystack needle (? from))
  haystack: a deque
  needle: any type
  from: an int (optional)
  returns an int or #n

Searches backwards within a deque.

rposition is identical to position, except the search proceeds backwards from the end of the deque.

The index returned is always a non-negative integer, so that it can more easily be compared to the indexes returned by position.

(let st "It bears the mark of Polaris.")

(prn (position st "ar")) ; prints 5
(prn (position st "ar" 10)) ; prints 14
(prn (rposition st "ar")) ; prints 24
(prn (rposition st "ar" 10)) ; prints 5