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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def append_warning(self, msg):
 """Append a warning message to the model to expose issues."""

        assert self.model is not None, "You must already have run make_model!"
        addendum = ('\t<span style="color:red;">(CAUTION: %s occurred when '
                    'creating this page.)</span>' % msg)
        self.model = self.model.replace(self.title, self.title + addendum)
        return self.model