For #19

This is the first shot at the new merge function. I didnt wanna rewrite the previous before we agree on this, since I think there will be some changes. Therefore I would like to discuss this first. Since we agreed not to do is as previously, ergo first scan everything and then merge or add, I've tried to do these function recursive. Haven't tested it much, I would like to agree if this would be a good way on how to write it.

Added correction for area_s as well as requested by Romain