NSMutableDictionary categories documentation
- - (void) addObject: (id)anObject forKey: (id)aKey
Adds an object for the specific key. If there is no value for this key, it is added. If there is an existing value and it is a mutable array, then the object is added to the array. If it is not a mutable array, the existing object and the new object are both added to a new array, which is set for this key in the dictionary.