How do I extend multiple interfaces in aura:application.

I want to use ltng:out and force:slds in the same app and expose the app on a public website. As a workaround I downloaded the slds class into static resource referenced through ltng: require for now

When I try to save this code in developer console:

<aura:application extends="force:slds,ltng:outApp" access="GLOBAL">
    <aura:dependency resource="c:casesearch"/>
</aura:application>

I get the following error: