Point taken, Michael.  Its a reasonable assumption to make
when there are many choices available to a developer of a
finite state machine.

Thank you (and Michael Glavassevich) for the answers.

Arshad Noor
StrongAuth, Inc.

Michael Kay wrote:
>> Given that A is optional, shouldn't the error message 
>> indicate that (at a minimum) "B or C" is expected?
>>
> 
> Designing good error messages tends to involve guessing what the user's
> intention was. The message that this particular processor has given you is
> what comes out easily from the usual strategy of compiling the grammar into
> a finite state machine, where in any given state you know what symbols are
> allowed to appear to move you to the next state. 
> 
> There are strategies that do better (spelling checkers use them, essentially
> by trying to work out what valid construct your invalid construct is closest
> to) but they tend to involve look-ahead, which means they don't scale well
> when the content is very long. And as we know from spelling checkers, they
> can sometimes produce hilarious results.
> 
> Michael Kay
> http://www.saxonica.com/
>