// the old version for reference

                                       StringTree * insert(StringTree * t, string s)
                                       { if (t==NULL)
                                           t = new StringTree(s);
                                         else if (s <= t->data)  
                                           t->left = insert(t->left, s);
                                         else
                                           t->right = insert(t->right, s);
                                         return t; }

                       // the reference parameter version for reference

                       void insert(StringTree * & t, string s)
                       { if (t==NULL)
                           t = new StringTree(s);
                         else if (s <= t->data)
                           insert(t->left, s);
                         else
                           insert(t->right, s); }


...
void insert(StringTree * * t, string s)
{ if (*t == NULL)
    *t = new StringTree(s);
  else if (s <= (*t)->data)
    insert(&((*t)->left), s);
  else
    insert(&((*t)->right), s); }
...
...
    if (command=="i")
    { string operand;
      cin >> operand;
      insert( & tree, operand);
      cout << operand << " inserted\n"; }
...