mymodule_content_build_modes overwrites existing build modes instead of concatenating

The description of the function node_content_build_modes() explains that

// A module can add its render modes to a tab defined by another module.

However, the following code will overwrite the existing build modes:

function mymodule_content_build_modes() {
  return array(
    'basic' => array(
      'title' => t('Basic'),
      'build modes' => array(
        'mymodule_mode1' => array(
        'title' => t('My mode 1'),
        'views style' => TRUE,
        ),
      ),
    ),
  );
}