code: func (l *List) Add(c Capability, values ...string) error {
  	if err := l.validate(c, values); err != nil {
  		return err
  	}
  
  	if !l.Supports(c) {
  		l.m[c] = &entry{Name: c}
  		l.sort = append(l.sort, c.String())
  	}
  
  	if len(values) == 0 {
  		return nil
  	}
  
  	if known[c] && !multipleArgument[c] && len(l.m[c].Values) > 0 {
  		return ErrMultipleArguments
  	}
  
  	l.m[c].Values = append(l.m[c].Values, values...)
  	return nil
  }
docs: // Add adds a capability, values are optional