code: public SuffixTrieNode<I> add(I symbol, SuffixTrieNode<I> parent) {
          SuffixTrieNode<I> n = new SuffixTrieNode<>(symbol, parent);
          if (nodes != null) {
              nodes.add(n);
          }
          return n;
      }
docs: Adds a word to the trie.
  
  @param symbol
  the first symbol of the word.
  @param parent
  the remaining suffix of the word.
  
  @return a trie node corresponding to the inserted word.