Refactoring: Abstract signature, Move internal class into point of use.
Refactoring: Extract common method.
Javadoc and refactoring test of flatMap.
Implement parallel flatMap operator.
Refactoring factory classes.
Refactoring: threshold considers both number of core and size of source.
Refactoring: use Sequence operator instead of loop.
Add javadoc and tests for groupBy operator.
Add javadoc and tests for partitionBy operator.
Refactoring test.
Use assert for internal check.
Refactoring Range.
Add tests.
Remove redundant class.
Remove unnecessary method.
Refactoring factory.
Correct test: Longer timeout.
Use factory method instead of constructor.
Implement parallel groupBy operator.
Change Grouping to List tagged with key.