How can I obfuscate only com.foo.* but excluding com.foo.bar.*? (Proguard)

I tried:

-keep class !com.foo.** { *; }
-keep class com.foo.bar.** { *; }

but it obfuscates classes other than com.foo.* (which I don't want)

I also tried:

-keep class !com.foo.**, com.foo.bar.** { *; }

and it fails to parse.