Delete unused class.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
Allow null values with merge function to implement reverse of
accumulation.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
Add convenience sort method using key extractor.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
Improve set operations.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
Improve error message when conflicting values  for map key detected.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
More intuitive parameter order for toMultiMap conversion.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>
Improve code for collecting to map.

Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>