def bbox(self):
xmin = min(self.start.real, self.end.real)<EOL>xmax = max(self.start.real, self.end.real)<EOL>ymin = min(self.start.imag, self.end.imag)<EOL>ymax = max(self.start.imag, self.end.imag)<EOL>return xmin, xmax, ymin, ymax<EOL>
returns the bounding box for the segment in the form
        (xmin, xmax, ymin, ymax).