Replace rectangle with its finite version.

def normalize(self):
        """Replace rectangle with its finite version."""
        if self.x1 < self.x0:
            self.x0, self.x1 = self.x1, self.x0
        if self.y1 < self.y0:
            self.y0, self.y1 = self.y1, self.y0
        return self