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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 subtract(self):
 """Subtract the given MOC from the running MOC. This command takes the name of a MOC file to be subtracted from the running MOC. :: pymoctool a.fits --subtract b.fits --output difference.fits """


        if self.moc is None:
            raise CommandError('No MOC information present for subtraction')

        filename = self.params.pop()
        self.moc -= MOC(filename=filename)