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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 prefixed(self, prefix):
 """Context manager for parsing envvars with a common prefix."""

        old_prefix = self._prefix
        if old_prefix is None:
            self._prefix = prefix
        else:
            self._prefix = "{}{}".format(old_prefix, prefix)
        yield self
        self._prefix = old_prefix