Correct test.
Fix that permutations(0) does not return empty list.
Remove rotate operator because skip-concat-take or tail-and-drop can cover use case.
Correct test.
Add combinations operator.
Let the permutations operator returns empty sequence instead of exception.

Let the permutations operator returns empty sequence instead of exception.

Let the permutations operator returns empty sequence instead of exception.

Let the permutations operator returns empty sequence instead of exception.
Add javadoc and test of the pairwise operator.
Rename iterate(Depth|Breadth)First to traverseXXX which is more appropriate because the verb "traverse" implies "come and go".
Add javadoc and test of the traverse(Breadth|Depth)First operator.
Add javadoc and test of the toSet operator.
Add javadoc and test of the scan operator.
Change (left|right)OuterJoin signature.
Commonalize (left|right)OuterJoin implementation.
Prune away verbose overload of the join operator.
Prune away verbose overload of the groupJoin operator.
Move function to proper class.
Move internal functions to Folders module.
Add private modifier to hide implementation.
Add test.
Add javadoc and test of the insert operator.