code: func keyExistsFunc(b *Brain, used, missing *dep.Set) func(string) (bool, error) {
  	return func(s string) (bool, error) {
  		if len(s) == 0 {
  			return false, nil
  		}
  
  		d, err := dep.NewKVGetQuery(s)
  		if err != nil {
  			return false, err
  		}
  
  		used.Add(d)
  
  		if value, ok := b.Recall(d); ok {
  			return value != nil, nil
  		}
  
  		missing.Add(d)
  
  		return false, nil
  	}
  }
docs: // keyExistsFunc returns true if a key exists, false otherwise.