docs: public void merge(Mutation<E,K> m) {
          Preconditions.checkNotNull(m);
  
          if (null != m.additions) {
              if (null == additions) additions = m.additions;
              else additions.addAll(m.additions);
          }
  
          if (null != m.deletions) {
              if (null == deletions) deletions = m.deletions;
              else deletions.addAll(m.deletions);
          }
      }