<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def evaluate(self, x, y, flux, x_0, y_0, sigma):
 """Model function Gaussian PSF model."""


        return (flux / 4 *
                ((self._erf((x - x_0 + 0.5) / (np.sqrt(2) * sigma)) -
                  self._erf((x - x_0 - 0.5) / (np.sqrt(2) * sigma))) *
                 (self._erf((y - y_0 + 0.5) / (np.sqrt(2) * sigma)) -
                  self._erf((y - y_0 - 0.5) / (np.sqrt(2) * sigma)))))