code: def from_rel_ref(baseURI, relative_ref):
          
          joined_uri = posixpath.join(baseURI, relative_ref)
          abs_uri = posixpath.abspath(joined_uri)
          return PackURI(abs_uri)
docs: Return a |PackURI| instance containing the absolute pack URI formed by
          translating *relative_ref* onto *baseURI*.