How to use config.add_settings({'pyramid.includes': ...}) from included callable

When I created a default pyramid app from a cookie cutter, it resulted in an INI file with sections like this:

[app:myapp]
use = egg:myproject#myapp
pyramid.reload_templates = true
pyramid.debug_authorization = false
pyramid.debug_notfound = false
pyramid.debug_routematch = false
pyramid.default_locale_name = en
pyramid.includes = pyramid_debugtoolbar

Now I'm experimenting with adding these same settings in python code instead, using the Configurator object in __init__.py, and I find that the following appears to work the same:

config.include('pyramid_debugtoolbar')
config.add_settings({
    'pyramid.reload_templates'      : 'true',
    'pyramid.debug_authorization'   : 'false',
    'pyramid.debug_notfound'        : 'false',
    'pyramid.debug_routematch'      : 'false',
    'pyramid.default_locale_name'   : 'en',
    'pyramid.includes'              : 'pyramid_debugtoolbar',
    })

But when applying these setting in python, the first line config.include('pyramid_debugtoolbar') is required or it doesn't work. Yet, in the INI version, it's sufficient to set pyramid.includes = pyramid_debugtoolbar.

After Further Digging

Looking higher up the stack in my code, I found that the setting does work this way...

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application."""
    settings.update({'pyramid.includes':'pyramid_debugtoolbar'}) # SETTING HERE WORKS!
    with Configurator(settings=settings) as config:
        config.include(common_config)
        config.include('.routes')
        config.scan()

    return config.make_wsgi_app()

But NOT this way...

def main(global_config, **settings):
    """ This function returns a Pyramid WSGI application."""
    with Configurator(settings=settings) as config:
        config.add_settings({'pyramid.includes':'pyramid_debugtoolbar'}) # NO EFFECT!
        config.include(common_config)
        config.include('.routes')
        config.scan()

    return config.make_wsgi_app()

In the documentation for pyramid.config, I found this warning that I suspect is what I'm dealing with:

A configuration callable should be a callable that accepts a single argument named config, which will be an instance of a Configurator. However, be warned that it will not be the same configurator instance on which you call this method. The code which runs as a result of calling the callable should invoke methods on the configurator passed to it which add configuration state. The return value of a callable will be ignored.

In an effort to guess at the solution, I tried wrapping my config.add_settings(...) with various combinations of config.commit() and config.begin()/config.end(), and none of those worked either.

My Question:

How do I use config.add_settings(...) to set pyramid.includes? I want to do this in a common_config() callable that is included by multiple pyramid apps.