code: func (m Map) Keys() []string {
  	ks := make(map[string]struct{})
  	for k, _ := range m {
  		idx := strings.Index(k, ".")
  		if idx == -1 {
  			idx = len(k)
  		}
  
  		ks[k[:idx]] = struct{}{}
  	}
  
  	result := make([]string, 0, len(ks))
  	for k, _ := range ks {
  		result = append(result, k)
  	}
  
  	return result
  }
docs: // Keys returns all of the top-level keys in this map