SPARK Examiner -- visibility of Ada.Strings.Unbounded (and probably lots of other packages)

[Tim Rowe <spamtrap@tgrowe.plus.net>]

Now that a GPL version of SPARK Examiner has been announced, I suppose 
this group will be getting a flood of queries on how to use it. Well, I 
hope so, because that would indicate a large take-up -- good for 
software as a whole, and presumably what Praxis HIS and AdaCore are 
hoping for.

So let me try to get in ahead of the flood. I'm trying to put my Ada 
learning exercise into SPARK, and I'm using the evaluation version of 
Examiner (and the gnat compiler). I have
    with Ada.Strings.Unbounded;
at the start of a package file, and SPARK Examiner complains:
    1  with Ada.Strings.Unbounded;
            ^1
--- (  1)  Warning           :  1: The identifier Ada is either 
undeclared or not
            visible at this point.

I can't work out how to make Ada.Strings.Unbounded visible to examiner. 
Could somebody point me to what I'm missing, please?

Thanks!