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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 as_bel(self) -> str: """Return this fusion range as a BEL string."""

        return '{reference}.{start}_{stop}'.format(
            reference=self[FUSION_REFERENCE],
            start=self[FUSION_START],
            stop=self[FUSION_STOP],
        )