public function mappings($mappings = null) {
        if ($this->mappings === null) {
            $this->mappings = [];
        }

        if (is_array($mappings)) {
            $mappings = array_change_key_case($mappings);

            $this->mappings = array_replace(
                $this->mappings,
                $mappings
            );
            return $this;
        }

        return $this->mappings;
    }