Double curly brackets

Hello

I want double curly brackets in math mode

{{a}}

and I came up with this

<assign|avg|<macro|body|<around*|{|<space|-0.8spc><around*|{|<arg|body>|}><space|-0.8spc>|}>>>

This works ok, but if the brackets size increases vertically, e.g., if I put a fraction inside, then the spacing between the two brackets {{ increases.

Is there a way to do this better ?

Thanks
praveen