User-Agent: * Allow: / Disallow: /posts Disallow: /posts/ Disallow: /tags Disallow: /tags/ [...]
Some major crawlers support an Allow directive which can counteract a following Disallow directive. This is useful when you disallow an entire directory but still want some HTML documents in that directory crawled and indexed. While by standard implementation the first matching robots.txt pattern always wins, Google's implementation differs in that it first evaluates all Allow patterns and only then all Disallow patterns. (irrelevant in this case since Allow is the first, so google's special behavior does not apply here)
Allow: / directive overrides all the following disallows rendering
robots.txt effectively useless.
This is the reason all major search engines (bing, google, etc) crawl pages they are not supposed to, like post revisions.