Refactoring: Use factory.
Correct test.
Optimize toArray.
Fix typo.
Reimplement distinct operator.
Implement parallel distinct operator.
Correct parameter name.
Implement union operator.
Reimplement index splitter.
Add tests.
Add protected modifier.
Add test.
Refactoring.
Refactoring test.
Implement parallel union.
Reimplement except operator.
Add override of except operator which accept comparator.
Implement parallel except operator.
Fix test.
Reimplement zip operator and zipAll operator.