private function addArray(string $key, $value): self
    {
        $this->props[$key][] = $value;

        return $this;
    }
private function addArray(string $key, $value): self
    {
        $this->props[$key][] = $value;

        return $this;
    }
private function addArray ( string $ key , $ value ) : self { $ this -> props [ $ key ] [ ] = $ value ; return $ this ; }
add new child array

@param mixed $value
add new child array