public function declareVariables(array $statement)
    {
        if (isset($statement['data-type'])) {
            if ('variable' != $statement['data-type']) {
                return;
            }
        }

        foreach ($statement['variables'] as $variable) {
            if (isset($variable['expr'])) {
                if ('string' == $variable['expr']['type'] || 'empty-array' == $variable['expr']['type'] || 'array' == $variable['expr']['type']) {
                    continue;
                }
            }

            $this->increaseMutations($variable['variable']);
        }
    }