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# Forward difference

Here we use signature matching to bind both an entire array and a version missing the head. The Z- operator is a zip metaoperator with a minus to subtract the two lists pairwise. It's almost a shame to define difn, since the series and subscript are hardly longer than the call itself would be, and arguably more self-documenting than the name of the function would be.

```perl
sub dif(@array [$, *@tail]) { @tail Z- @array }
sub difn($array, $n) { ($array, &dif ... *)[$n] }
```