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<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /subdirectoryName
RewriteRule ^subdirectoryName/index\.html$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /subdirectoryName/index.html [L]
</IfModule>