Q:

Redirect all requests on subfolder to subfolder/index.html with .htaccess

At the moment I'm using this .htaccess:
<IfModule mod_rewrite.c>
    <IfModule mod_negotiation.c>
        Options -MultiViews
    </IfModule>

    RewriteEngine On

    RewriteRule ^((?!app/).*)$ /app/$1 [L,NC,R=301,NE]

    # Redirect Trailing Slashes...
    RewriteRule ^(.*)/$ /$1 [L,R=301]

    # Handle Front Controller...
    RewriteCond %{REQUEST_FILENAME} ! -d
    RewriteCond %{REQUEST_FILENAME} ! -f
    RewriteRule ^ index.php [L]
</IfModule>

I'm trying to get it so that anything starting with /app goes to the app folder, but when I hit /app/debates it gets handled by the index.php file.  If I hit /app then it does get routed correctly. 
How can I modify this to handle all sub-URLs too? 

A:

Insert this rule just below RewriteEngine On line:
RewriteRule ^app/ - [L,NC]

