Sorry, it's just a reference to the ruby-red control flow edge flowing through the intermediate representation graph nodes, like a root. The same line in the logo.

And it starts with R, so the idea was I could write a new command-line option -rhizome to enable it, but that would actually run a library hizome.rb, with -r being the standard option to load libraries in Ruby, so letting me add a new option to an unmodified Ruby.