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After further study of CLHS, I've concluded the statements

about branch cuts for ATANH are inconsistent with the

formula (log(1 + x) - log(1 - x))/2. The branch cuts as

derived from the formula are (-\infty, -1) continuous w/ 2nd

quadrant and (1, \infty) continuous w/ 4th. I think the only

way to read the spec for ATANH is to read the stuff about

branch cuts as commentary and ignore it (i.e., observe the

formula and ignore the commentary).

That said, there is still some strangeness in ATANH, namely

that (ATANH -2.5) => #C(-0.42364892 1.5707964) which is

different from (ATANH #C(-2.5 0.0)) => #C(-0.42364892

-1.5707964). Since Clisp doesn't recognize signed zero, I

don't see how that can be. For the record, (IMAGPART -2.5)

=> 0 and (PHASE -2.5) => 3.1415927.

In order to be consistent w/ (log(1 + x) - log(1 - x))/2,

(ATANH #C(-2.5 0.0)) should yield #C(-0.42364892 1.5707964).