code: @Nullable
      public Node<I> getChild(int idx) {
          if (children == null) {
              return null;
          }
          return children.array[idx];
      }
docs: Retrieves, for a given index, the respective child of this node.
  
  @param idx
  the alphabet symbol index
  
  @return the child for the given index, or {@code null} if there is no such child