CSE machine: inaccurate arrow from stash to pair

[martin-henz]

https://share.sourceacademy.org/vwwjn

The arrow should point to the first pair of xs, not between the first and the second pair.

const append =
      (xs, ys) => is_null(xs)
                  ? ys
                  : pair(head(xs), append(tail(xs), ys));

append(list(1,2), list(3));