Empty Set Symbol



I'm working in the sets theory... What i have to do if I want everytime 
there is a {} (empty set) in a result maxima shows a symbol (or a 
letter)? Example:

{{},{{}}}

I want maxima showing this:

{ES,{ES}}

(ES for Empty Set...but it's okay anyway..)

Please Help me!

It's very important...
Thanks!




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