code: def relative_ref(self, baseURI):
          
          # workaround for posixpath bug in 2.6, doesn't generate correct
          # relative path when *start* (second) parameter is root ('/')
          if baseURI == '/':
              relpath = self[1:]
          else:
              relpath = posixpath.relpath(self, baseURI)
          return relpath
docs: Return string containing relative reference to package item from
          *baseURI*. E.g. PackURI('/ppt/slideLayouts/slideLayout1.xml') would
          return '../slideLayouts/slideLayout1.xml' for baseURI '/ppt/slides'.