code: @Override
    public void setChildren(Tree[] children) {
      if (children == null) {
        System.err.println("Warning -- you tried to set the children of a SimpleTree to null.\nYou should be really using a zero-length array instead.");
        daughterTrees = EMPTY_TREE_ARRAY;
      } else {
        daughterTrees = children;
      }
    }
docs: Sets the children of this <code>Tree</code>.  If given
  <code>null</code>, this method sets the Tree's children to a
  unique zero-length Tree[] array.
  
  @param children An array of child trees