code: func (ps *PointSet) SetAt(index int, point *Point) *PointSet {
  	deref := *ps
  	if index >= len(deref) || index < 0 {
  		panic(fmt.Sprintf("geo: set index out of range, requested: %d, length: %d", index, len(deref)))
  	}
  	deref[index] = *point
  	*ps = deref
  	return ps
  }
docs: // SetAt updates a position at i in the point set