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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 restore_existing_iam_env_vars(self):
 """Restore backed up IAM environment variables."""

        for i in ['AWS_ACCESS_KEY_ID', 'AWS_SECRET_ACCESS_KEY',
                  'AWS_SESSION_TOKEN']:
            if 'OLD_' + i in self.env_vars:
                self.env_vars[i] = self.env_vars['OLD_' + i]
            elif i in self.env_vars:
                self.env_vars.pop(i)