'Return a |PackURI| instance containing the absolute pack URI formed by
translating *relative_ref* onto *baseURI*.'

@staticmethod
def from_rel_ref(baseURI, relative_ref):

    joined_uri = posixpath.join(baseURI, relative_ref)
    abs_uri = posixpath.abspath(joined_uri)
    return PackURI(abs_uri)
