code: @Override
    public Boolean sismember(final byte[] key, final byte[] member) {
      checkIsInMultiOrPipeline();
      client.sismember(key, member);
      return client.getIntegerReply() == 1;
    }
docs: Return true if member is a member of the set stored at key, otherwise false is returned.
  <p>
  Time complexity O(1)
  @param key
  @param member
  @return Boolean reply, specifically: true if the element is a member of the set false if the element
  is not a member of the set OR if the key does not exist