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[satrajit AT attbi.com]

I'm new to Coq. Here's what I need to do:

Definition A:B->B:=
    [b:B] Cases b of
         (foo b') => ......
         _        => _
    end.

However, I'm getting the following error at the last but one line:
Error: The reference _ was not found in the current environment.

I assume, the construct _ => _, which I noticed in a similar definition in a 
paper based on Coq V6.10, is no longer supported. What is the way out?

By the way, if the above information is not enough for a useful 
comment/solution, please let me know that too.