addString (string) {
    if (this.cs === false) {
      string = string.toLowerCase()
    }

    // If the string has only one letter, mark this as a word.
    if (string.length === 0) {
      const wasWord = this.$
      this.$ = true
      return wasWord
    }

    // Make sure theres a Trie node in our dictionary
    let next = this.dictionary[string[0]]

    if (!next) {
      this.dictionary[string[0]] = new Trie(this.cs)
      next = this.dictionary[string[0]]
    }

    // Continue adding the string
    return next.addString(string.substring(1))
  }
Add a single string to the TRIE, returns true if the word was already in the
trie.