I am building a persistent array using a variant of rope-data-structure.
The array is actually held in a tree and there is a choice of using
Arc to hold child references.
Rc is good for performance, but do not have
Sync but not as good as
Rc in performance.
I don't want to repeat the entire implementation, one for
Rc, and another for
Arc. I don't know how best to handle this situation.