<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cv_residuals(self, cv=True):
 """Return the residuals of the cross-validation for the fit data"""

        vals = self.cv_values(cv)
        return (self.y - vals) / self.dy