Fix an rdeque
Maintains the invariant that there is always something in two stacks used by rdeques under the hood so long as there is 2 more elements in the rdeque.
rdeque to fix.
additional arguments to be passed to or from methods (ignored).
In fact, fix will be called whenever there are fewer than 6 elements in both the front and end of the deque. Generally this method is O(N), and so a full copy is returned.
fixed, "balanced" deque.
Okasaki, Chris. Purely Functional Data Structures. Cambridge University Press, 1999.