Yaml access to an array (Symfony 2)

Who can faced. There is a bundle with parameters.yml

parameters.yml
-

parameters: acme_default.options: constants: root_path: /home/www/acme.ru/web acme_default.config_path: %acme_default.options.constants[root_path]%/cfg.ini 

Such a thing does not work. Question: How can I get to the yml-file to the parameter (root_path), which lies in the array (constants)?