code: func (b *rangeBuilder) Range() (*Range, error) {
  	if b.err != nil {
  		return nil, b.err
  	}
  	if b.r.from != nil && b.r.to != nil && *b.r.to < *b.r.from {
  		return nil, ErrInvalidRange
  	}
  	return &b.r, nil
  }
docs: // Range returns the completed Range from the rangeBuilder.