> I was thinking about about a case where paths are resolved relative 
> to a container root in a filesystem. 

I can see the need for generalized 'drive' support that sets an arbitrary path prefix as the 'drive'. For example, if "/var/tmp/instroot" is a 'drive', then joining it to "/some/path" returns "/var/tmp/instroot/some/path". However, subsequently joining that result to "/suffix" would return "/var/tmp/instroot/suffix". The "/some/path" part is replaced by "/suffix". This doesn't match your example to lstrip('/') from paths joined with the proposed `//` operator.