Also:  -1.0 shouldn't be returned at this level to indicate an error;  these are pure C functions 
we're writing---the Python wrappers, and Python error conventions, apply further up the chain 
somewhere.

Just so I'm doing something a little more constructive than yelling criticisms from the sideline,  
I've attached a file invhyp.c that's how I think the C functions should look.  Some notes:

- it doesn't touch errno, but lets the platform decide how to handle errors (i.e. produce a 
special value/set errno/signal a floating-point exception/some combination of these).  This will 
make the asinh, acosh, atanh functions behave in the same way that the regular libm functions 
behave on any platform.  So e.g. if a particular platform is used to setting errno for domain 
errors like sqrt(-1), it'll do so for atanh/asinh/acosh.  And another platform that signals a 
floating-point exception for sqrt(-1) will do the same for atanh(3).

- I've left in the "huge+x > 1.0" test in asinh;  it's kinda pointless, but also fairly harmless.  
I think its only point is to make sure that the inexact flag gets set where appropriate, and 
Python doesn't much care about the inexact flag.

- I'm reasonably sure that the test "x == 1." in acosh is safe.