What does double slash (//) directory mean in robots.txt?

You will get the following output with:

curl https://www.ibm.com/robots.txt 

I delete many lines, keeping only part of it.

User-agent: * Disallow: // Disallow: /account/registration Disallow: /account/mypro Disallow: /account/myint  # Added to block site mirroring User-agent: HTTrack Disallow: / # 

I understand that / means root directory, but what does double slash // directory mean here in robots.txt?