In this case, all **partial sums** are needed to print the total stock quantities of a given stock_article after each stock_change. Using `inject` I only can think of (untested!)
```ruby
psums = self.inject([0]) {|psums, val| psums << psums.last+val}
psums.shift
psums
```

which would be less readable than the `map`, right? Should I rename the method to `partial_sums`?