If the suit breaks
3-2 there is a loser whatever sensible line you take. If the suit
breaks 5-0 you've got problems.
So concentrate on the 4-1 breaks.
Cashing the ace
wins when either opponent has a singleton king, but running the jack
wins when West has a singleton ten, nine or seven.
If South's suit
were AQ6432 cashing the ace would be best.