docs: function sdiff(key /* key-1, key-N, req*/) {
    var args = slice.call(arguments, 1)
      , req = typeof args[args.length - 1] === 'object' ? args.pop() : null
      , set = this.getKey(key, req)
      , list
      , src
      , j
      , i;
  
    if(!set) return [];
  
    list = set.data.slice(0);
  
    for(j = 0;j < list.length;j++) {
      for(i = 0;i < args.length;i++) {
        src = this.getKey(args[i], req);
        if(src === undefined) continue;
        if(src.sismember(list[j])) {
          list.splice(j, 1);
          j--;
          break;
        }
      }
    }
  
    return list;
  }