def save(root_node, filename)
      # TODO: - accept any I/O object?
      stringify_tree_from root_node
      File.open(filename, 'w') {|f| f << @sgf}
    end
def save(root_node, filename)
      # TODO: - accept any I/O object?
      stringify_tree_from root_node
      File.open(filename, 'w') {|f| f << @sgf}
    end
Takes a node and a filename as arguments