RPN Tutorial Exercise 4
-----------------------

Either `( 2 , 3 )' or `( 2 SPC 3 )' would have worked, but using both
the comma and the space at once yields:

     1:  ( ...      2:  ( ...      1:  (2, ...    2:  (2, ...    2:  (2, ...
         .          1:  2              .          1:  (2, ...    1:  (2, 3)
                        .                             .              .

         (              2              ,             SPC            3 )

Joe probably tried to type `TAB DEL' to swap the extra incomplete
object to the top of the stack and delete it.  But a feature of Calc
is that DEL on an incomplete object deletes just one component out of
that object, so he had to press DEL twice to finish the job.

     2:  (2, ...    2:  (2, 3)     2:  (2, 3)     1:  (2, 3)
     1:  (2, 3)     1:  (2, ...    1:  ( ...          .
         .              .              .

                       TAB            DEL            DEL

(As it turns out, deleting the second-to-top stack entry happens often
enough that Calc provides a special key, `M-DEL', to do just that.
`M-DEL' is just like `TAB DEL', except that it doesn't exhibit the
"feature" that tripped poor Joe.)