I use robots.txt to block all robots.txt compliant bots from a directory like this:
User-agent: * Disallow: /sub/*
How can I override this so to allow Mediapartners-Google (AdSense) to /sub/?
Adding this at the top of the robots.txt works:
User-agent: Mediapartners-Google Disallow:
N.B. you may need AdSense to download your robots.txt again, which I believe they do on a 24hr basis.