Rewrote keys with explicit recursion
Rewrite t_foldl'. It was not testing anything.
Rewrite t_unionR using ===
Rewrite t_deleteMin using ===
Rewrite t_deleteMax using ===
Rewrite t_insert_present and t_insert_missing with ===
Differences implemented
Code simplified
Merged with intersection
intersectionWithKey and differenceWithKey unified
Error messages fixed
Flesh out the Set type a little
Remove redefinition of Control.Arrow.first.
Remove trailing whitespace.
Remove trailing whitespace :-(
Remove redundant type witnesses
Simplify t_insertWith_present and t_insertWith_missing
Corrected excessively lazy benchmarks.
Corrected delete/view min/max functions.

They had been leaving Empty internal nodes in the tree.
Corrected updateExtremity-based functions.

They had been leaving Empty internal nodes in the tree.