docs: public void put(String key, String value) {
          Preconditions.checkNotNull(key, "key is null");
          Preconditions.checkNotNull(value, "value is null");
          Preconditions.checkArgument(key.indexOf('/') == -1, "key must not contain a slash");
          Preconditions.checkArgument(!children.containsKey(key), "key identifies a path, put operation not allowed");
          values.put(key, value);
      }