Refactoring: Hide internal usage of Tuple.
Refactoring: Rename internal abstract class.
Solve problems reported by code analysis.
Clean up javadoc.
Add test.
Reimplement intersect operator.
Correct test.
Override intersect operator that use key comparator.
Correct test.
Implement parallel union operator.
Correct javadoc.
Reimplement intersect operator.
Reimplement sequenceEqual operator.
Implement sequenceEqual operator.
Implement toString & toJoinedString operator.
Implement forEach operator.
Fix test.
Refactoring filter oeprator.
Allow null in parallel sequence.
Implement toString operator of parallel sequence.