text: export function zscore(key, member) {
    const map = this.data.get(key);
  
    // @TODO investigate a more stable way to detect sorted lists
    if (!map || !(map instanceof Map)) {
      return null;
    }
  
    const entry = map.get(member);
  
    if (!entry) {
      return null;
    }
  
    return entry.score.toString();
  }
  