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#### 2.7.4 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.)