> An open tool is Internet email: I can send an email from my provider
> (ucop.edu) to yours (princeton.edu). A closed tool is github, where I
> need a github account to send you a pull request. An open tool would
> be one where I can send a pull request bitbucket to github.
> (Obviously, bitbucket is as closed as github in this regard.)
> 
> best, Erik
> Sent from my free software system <http://fsf.org/>.

Uhh… That's a different definition of "closed system" than I'm used to seeing. It's more akin to having a closed-source e-mail client vs. an open-source one. The protocol (git) is open. I can push, pull, merge, fork, and do everything I need to do with a repository without ever visiting GitHub -- even pushing and pulling to/from pull requests. I can send you a patch that you can apply on your Bitbucket repo without needing to touch GitHub. I can even do multiple origins so that I can pull from GitHub but push to Bitbucket, and vice versa. So while the tool may technically be closed-source, the protocol--the equivalent to SMTP, IMAP, and POP in your example--is wide open.

Heck, even Richard Stallman gives GitHub a pass: "I use the term "SaaS" for services that do your computing for you, but not for services that do only communication. Thus, gmail.com is not SaaS. Wordpress is not SaaS. Github is not SaaS, or perhaps only in trivial ways." (https://mayfirst.org/lowdown/august-2011/richard-m-stallman-lectures-free-software-west-bank)

GitHub makes a lot of things about using git collaboratively *easier*, but I can do everything I need to on a collaborative project without ever visiting the GitHub page itself, provided I don't want to look at the issue tracker or wiki.  The "Pull Request" button is just a shortcut to merging two remote origins. If you wanted to make a system where you can send pull requests to GitHub and Bitbucket, you can and nobody will stop you.

See also: http://gitlab.org.