'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):

    if (baseURI == '/'):
        relpath = self[1:]
    else:
        relpath = posixpath.relpath(self, baseURI)
    return relpath
