If the pending count is nonzero, decrements the count; otherwise invokes {@link #onCompletion(CountedCompleter)} and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.
See Implementation
If the pending count is nonzero, decrements the count; otherwise invokes {@link #onCompletion(CountedCompleter)} and then similarly tries to complete this task's completer, if one exists, else marks this task as complete.