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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 get_dev_at_mountpoint(mntpoint):
 """ Retrieves the device mounted at mntpoint, or raises MountError if none. """

        results = util.subp(['findmnt', '-o', 'SOURCE', mntpoint])
        if results.return_code != 0:
            raise MountError('No device mounted at %s' % mntpoint)

        stdout = results.stdout.decode(sys.getdefaultencoding())
        return stdout.replace('SOURCE\n', '').strip().split('\n')[-1]