Did I correctly prune this min-max search tree using alpha-beta pruning?
I once implemented this in common lisp and running my program on your tree tells me that the pruned nodes are the one you marked and also the rightmost leaf and its parent.  The output of the program gives $\alpha$ and $\beta$ values for each node (if you mark from A through X in a BFS manner, left-to-right.  Then O and J are pruned).

MIN: K     -INFTY    11        [v:11]
MIN: L     11        INFTY     [v:9]
MAX: E     11        INFTY     [v:11]
MIN: M     -INFTY    4         [v:4]
MIN: N     4         11        [v:13]
MAX: F     4         11        [v:13]
MIN: B     -INFTY    11        [v:11]
MIN: P     11        12        [v:12]
MIN: Q     12        INFTY     [v:8]
MIN: R     12        INFTY     [v:4]
MAX: G     12        INFTY     [v:12]
MIN: C     11        12        [v:12]
MIN: S     12        INFTY     [v:9]
MIN: T     12        13        [v:13]
MIN: U     13        INFTY     [v:1]
MAX: H     13        INFTY     [v:13]
MIN: V     12        13        [v:12]
MIN: W     12        13        [v:9]
MAX: I     12        13        [v:12]
MIN: D     12        13        [v:12]
MAX: A     12        INFTY     [v:12]
12 
21 
T