Enumerates the supported modes of merging the results of requests. If keys (A, B) mapped to results ([1,2,4],[3,4,5])...
- UseFirst (or None) takes the results from the first key -- [1,2,4]
- Union takes results of each key and collects items that matched any key. -- [1,2,3,4,5]
- Intersection takes results of each key, and collects items that matched every key. -- 
public enum MergeMode