public function addSubstitutions(
        $substitutions,
        $personalizationIndex = null,
        $personalization = null
    ) {
        if (current($substitutions) instanceof Substitution) {
            foreach ($substitutions as $substitution) {
                $this->addSubstitution($substitution);
            }
        } else {
            foreach ($substitutions as $key => $value) {
                $this->addSubstitution(
                    $key,
                    $value,
                    $personalizationIndex,
                    $personalization
                );
            }
        }
    }