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.
Usage
fixd(d, ...)
Arguments
d
rdeque to fix.
...
additional arguments to be passed to or from methods (ignored).
Value
fixed, "balanced" deque.
Details
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.
References
Okasaki, Chris. Purely Functional Data Structures. Cambridge University Press, 1999.