'Return a tuple: (the correct value of based_on, whether the old
value was correct).
based_on must be a revision of the English version of the document. If
based_on is not already set when this is called, the return value
defaults to something reasonable.'

def _based_on_is_clean(self):

    original = self.document.original
    if (self.based_on and (self.based_on.document != original)):
        return (original.localizable_or_latest_revision(), False)
    return (self.based_on, True)
