code: def next(self):
          
          while True:
              x, y = next(self.scan)
              self.index += 1
              if (self.index < self.start): continue
              if (self.index > self.stop): raise StopIteration("skip stopping")
              if ((self.index-self.start) % self.step != 0): continue
              return x, y
docs: Next point in iteration