Revert changes to atanh
Still need the special case for real args to get the right branch cuts. Haven't yet figured out why the current code doesn't work when the arg is a pure real. Soemthing to do with computing y, but not sure how to get that to work out without doing just explicitly checking for real args. In that case, it's just much easier to change the arg to have the correct imaginary component to make it continuous with the desired quadrant.
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