Write a function cycle-lists that takes two lists, xs and ys, and returns a stream. The lists may or may not be the same length, but you may assume they are both non-empty. The elements produced by the stream are pairs where the first part is from xs and the second part is from ys. The stream cycles forever through the lists. Your solution should not require the use of mutable state, i.e., it should not use set! . A constant-time ( O(1)) solution is possible. Examples: (stream-take-n 8 (cycle-lists '(1 2 3) '("a" "b"))) => ((1. "a") (2. "b") (3. "a") (1. "b") (2. "a") (3. "b") (1. "a") (2. "b")) Hint: Think about how you could create a stream that endlessly cycles through all the elements of a single, given list. You can use a local environment to store a copy of the list through which you must cycle and a helper function that uses this stored copy when it runs out of elements to cycle through.