def add_hook(name = nil, cascade = false, &hook)
    if name == nil or name == true or name == false
      cascade = name
      assign_self_hook(cascade, &hook)
    else
      assign_hook(name_to_a(name), cascade, &hook)
    end
  end
def add_hook(name = nil, cascade = false, &hook)
    if name == nil or name == true or name == false
      cascade = name
      assign_self_hook(cascade, &hook)
    else
      assign_hook(name_to_a(name), cascade, &hook)
    end
  end
name: a Symbol, String or an Array; nil means hook to the root
 cascade: true/false; for cascading hook of sub key
 hook: block which will be called with 2 args, name and value