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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 protein modification variant as a BEL string."""

        return 'pmod({}{})'.format(
            str(self[IDENTIFIER]),
            ''.join(', {}'.format(self[x]) for x in PMOD_ORDER[2:] if x in self)
        )