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(quoted from Darren)

...
  cross([1,2],[3]); # yields ([1,3],[2,3])
  cross([1],[2],[3]) # yields ([1,2,3])
  cross([1],[2],[3,4]) # yields ([1,2,3],[1,2,4])

sub cross {
    my @list = @_.reverse or return;
...
        }
    }
}