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'.

def relative_ref(self, baseURI):
        """
        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'.
        """
        # 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