Make operations nil-safe: if any of arguments to the combiner functions are nil,  return nil. It will be skipped on the last aggregation step when returning results.
Make operations nil-safe: if any of arguments to the combiner functions are nil, 
return nil. It will be skipped on the last aggregation step when returning results.
zbintliff/cyanite,pyr/cyanite,zbintliff/cyanite,pyr/cyanite,pyr/cyanite