"Kill" text in a "rectangle" and save its contents

    C-x r k

Switch to the scratch buffer (think there's a missing RET at the end)

    C-x b *scr C-I

"Yank" the "rectangle" saved in the first command

    C-x r y

Moves to the start of the buffer and sets a "mark"

    M-<

Literals that are entered at the start of the buffer

    (+

Move to the end of the buffer

    M->

Another literal

    )

Executes the buffer (or at least the LISP expression at the cursor). Note the scratch buffer allows you to type arbitrary LISP stuff and execute it with C-j.

    C-j

So, suppose the initial buffer contained

    1
    1
    2
    3

Then the scratch buffer would get turned into

    (+1
    1
    2
    3 )

And when executed this would add the list of numbers and look like this:

    (+
    1
    1
    2
    3)
    7

Another way to sum a column of numbers uses calc:

  M-x calc-grab-rectangle RET V R +