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