Brackets to define a mob don't work with /kill, /tp, /effect,..

Brackets to define a mob don't work with /kill, /tp, /effect,..

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/testfor @e[...] {...}

does work, but for example

/kill @e[...] {...}

does not.

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More examples:

/testfor @e[type=Slime] {Riding:{id:Zombie}}

is working.

/kill @e[r=3,c=-1,type=Slime] {Riding:{id:Zombie}}

is not.

Making this work would help me to create a headshot detection system for my awesome soon to be playable 'Tower Defense' style game.

Also i think it would be nice to have a RiddenBy-Tag to see if another entity is riding on the specific entity I want to be checked or manipulated.
(Because I want to kill the ZOMBIES that have NO invisible slime on their head)

I think a lot of people could make a use of this.

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I have created a reddit post for this, but it does not feel like this issues are getting upvoted much because it does only affect map makers and not normal players (which are most of the people obviously).

http://redd.it/1wdrwu

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Edit: Used proper formatting. Thanks to [Mod] Talven81.

Edit2: Can you think of any other way to kill a zombie that has no slime riding on it, without the above suggestions (RiddenBy-Tag for example)?
Think of the multiple options this would give to the players!

Edit3: How can I reopen this issue? How could I verificate the answer?