搜索网页中的链接，关注它们，并返回R中没有链接的页面

I am writing an update to my rNOMADS package to include all the models on the NOMADS web site. To do this, I must search the html directory tree for each model. I do not know how deep this tree is, or how many branches it contains, beforehand. Therefore I am writing a simple web crawler to recursively search the top page for links, follow each link, and return the URLs of pages that have no more links. Such a page is the download page for model data. to