Detecting whether a module was imported or not

[CarlosH]

In my module, I'd like to do something like
    
    
    when defined(nre):
      import nre
      # code that works with regular expressions
    

so, if the user does 
    
    
    import nre
    import my_module
    

then that code gets compiled, but if the user doesn't use nre, then we aren't 
going to uselessly import it or compile the related code in my_module.

Is something like that possible?