Imported current version from 05-Structured.
Introduced ThreadLocal.
Method reference.
DfPair renamed.
Added recalculation on commit from different thread.
Making the solution thread-safe.
Fixes to implementation.
Concurrency tests added.
Merge remote-tracking branch 'upstream/master' into parallel
Updated to reflect changes in the required interface.
Check for empty arguments.
Updated tests.
Fixed sync().
Fixed null check.
Merge pull request #463 from Norrius/parallel

Захаров Андрей, 395, Parallel