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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 save_signal(self,filename=None):
 """ Saves TransitSignal. Calls :func:`TransitSignal.save`; default filename is ``trsig.pkl`` in ``self.folder``. """

        if filename is None:
            filename = os.path.join(self.folder,'trsig.pkl')
        self.trsig.save(filename)