code: func series(f func(float64) float64) float64 {
  	y, yp := 0.0, 1.0
  	for n := 0.0; y != yp; n++ {
  		yp = y
  		y += f(n)
  	}
  	return y
  }
docs: // series returns the sum of the series f(0), f(1), ...
  //
  // This implementation is fast, but subject to round-off error.