code: public function add(...$param) : ToStringHelper
      {
          $count = count($param);
          Preconditions::checkArgument($count == 1 || $count == 2);
          $this->values[] = $count == 2
              ? new ToStringHelperItem($param[0], $param[1])
              : new ToStringHelperItem(null, $param[0]);
          return $this;
      }
docs: Can be called with 1 or 2 parameters. In the first case only the value,
  otherwise a key/value pair is passed.
  
  @param mixed[] $param
  @return ToStringHelper $this
  @throws \InvalidArgumentException if the number of parameters is not 1 or 2