What is the correct way to add a new element to this type of dictionary (deep dictionary) in Swift 5:

```
var myDictionary : [String : [String : [String]]] = [:]
for (word: String,type: String,sentences: [String]) in collection {
  myDictionary[word] = [type : sentences] // assignment
  // but what if myDictionary[word] already existed? and if type already existed?
  // myDictionary[word, default: [type : sentences]] // ???
}

```