def node_start(node)
      @node = node
      self.class.get_callbacks("start_#{node.sexp_type}".to_sym).each do |block|
        self.instance_exec(node, &block)
      end
    end
def node_start(node)
      @node = node
      self.class.get_callbacks("start_#{node.sexp_type}".to_sym).each do |block|
        self.instance_exec(node, &block)
      end
    end
delegate to start_### according to the sexp_type, like

     start_call
     start_def

 @param [Sexp] node