code: func (s *children) truncate(index int) {
  	var toClear children
  	*s, toClear = (*s)[:index], (*s)[index:]
  	for len(toClear) > 0 {
  		toClear = toClear[copy(toClear, nilChildren):]
  	}
  }
docs: // truncate truncates this instance at index so that it contains only the
  // first index children. index must be less than or equal to length.