def isperpendicular(self, pt1, pt2, pt3):
            """
            Test a set of points to see if they are perpendicular.
            Points are deemed perpendicular if they describe two 
            different lines that are parallel to the X and Y axes.

            @param pt1: The first point to test.
            @param pt2: The second point to test.
            @param pt3: The third point to test.

            @return: A boolean flag indicating if the points describe
            a set of perpendicular lines that are parallel with the 
            X and Y axes.
            """
            dy1 = pt2.coords[1] - pt1.coords[1]
            dx1 = pt1.coords[0] - pt2.coords[0]
            dy2 = pt3.coords[1] - pt2.coords[1]
            dx2 = pt3.coords[0] - pt2.coords[0]

            if abs(dx1) == 0 and abs(dy2) == 0:
                return False
            elif abs(dy1) == 0:
                return True
            elif abs(dy2) == 0:
                return True
            elif abs(dx1) == 0:
                return True
            elif abs(dx2) == 0:
                return True

            return False
dataset/ETHPy150Open PublicMapping/DistrictBuilder/django/publicmapping/redistricting/calculators.py/Roeck.Circle.isperpendicular/swapped operands of `-`