hide the root instance?

The only option that I could think of is to utilise the "can be root" constraint:

concepts constraints Library { 
  can be child <none> 
   
  can be root 
    (operationContext, model)->boolean { 
      model.roots(MyRootConcept).isNotEmpty; 
    } 
   
  can be parent <none> 
   
  can be ancestor <none> 
   
   
  <<property constraints>> 
   
  <<referent constraints>> 
   
  default scope 
    <no default scope> 
   
}

You, however, have to make a decision based on the containing model only, which might be quite difficult.

looks like the explanation is interpreted wrong.
When we click on Model in Sandbox, we get root instances to add. My requirement is to hide those instances.

Fox example, i have added baseLanguage to my usedLanguages in sandbax. Now when we right click on Model we get baseLanguage instance to add.
I want to hide it.

I don't think you can do that.

Here's the answer http://forum.jetbrains.com/thread/Meta-Programming-System-2751. But note that this feature will be available in upcoming 3.2.1 and it is not made to be extendable