in reply to greatest common factor

I found this function and it seemed reasonable, but some of my users reported that I was somtimes returning the wrong sign.
Turns out that the function above has inconsistent behavior on negative numbers:

when they should really both be 5.gcf(-5, 5) = 5 gcf(5, -5) = -5

14 years late, but here's a fix. I changed my code so that `gcf()` ends with

`return abs $x;`

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