code: func NewNexter(opts ...NexterOption) *Nexter {
  	var id uint64
  	n := &Nexter{
  		id: &id,
  	}
  	for _, opt := range opts {
  		opt(n)
  	}
  	return n
  }
docs: // NewNexter creates a new id generator starting at 0 - can be modified by
  // options.