My absolute favorite example of what's possible with awk is this[0] calculator from Ward Cunningham about splitting expenses on a ski trip. It's a really beautiful little piece of code well-adapted to this problem.
> The first occurrence of a variable name defines it as that sum. Subsequent occurrences become the stored value.
With this quote in mind - why is second `SUM` reference replaced with `-138.95` and not `221.81` (stored value) ?
EDIT: Never mind, now I see it's an exception on line 3.