code: public void setChild(int idx, int alphabetSize, Node<I> child) {
          if (children == null) {
              children = new ResizingArrayStorage<>(Node.class, alphabetSize);
          }
          children.array[idx] = child;
      }
docs: Sets the child for a given index.
  
  @param idx
  the alphabet symbol index
  @param alphabetSize
  the overall alphabet size; this is needed if a new children array needs to be created
  @param child
  the new child