How to hide the mail backend in the groups buffer?

[Peter Wu]

I use gnus to handle emails. The group buffer looks like this.

         0: nndraft:drafts 
         0: nndraft:queue 
         0: nnfolder+archive:sent 
         0: nnmaildir:inbox 


How can I hide the mail backends so that the buffer will look like:

         0: drafts 
         0: queue 
         0: sent 
         0: inbox 

TIA.

-- 
Peter Wu
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