src/precore/util/ToStringHelper.php
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 $this;
    }
public function add(...$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;
    }
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