Also, for the C-level routines, atanh(1.0) and atanh(-1.0) should definitely return 
infinity and -infinity (and probably set errno as well.)

Note that this is not an argument about what Python should do:  Python will still raise a 
ValueError for atanh(1.0) and atanh(-1.0).  But the atanh is supposed to be a drop-in 
replacement for the libm atanh, on those platforms where it's missing.  And the C99 
standard is clear about return values, even though it's not useful when it comes to 
deciding whether to set errno or not.