def checkOverlap(self):
    x = None
    for p in self.points:
        if (p[0] <= x):
            raise ValueError('overlapping points')
        x = p[0]
def checkOverlap(self):
    x = None
    for p in self.points:
        if (p[0] <= x):
            raise ValueError('overlapping points')
        x = p[0]