http://apt.include-once.org/

Add this repository to /etc/apt/sources.list as:

deb http://apt.include-once.org/ ./ #include-once.org

Import verification key with:

wget -q http://apt.include-once.org/public.gpg -O- | sudo apt-key add -

Packages here are auto-signed. No guarantees regarding unmodified sources; and possibly user-supplied content.

Btw, don't just trust commandline examples from arbitrary websites.
Check with your help system whenever you can't verify or remember if it's a harmless action.