> The macro Py_IS_INFINITY don't work on linux. The test case(force-inf.c)
> is attached. The result don't depend from optimisation flag.

Thanks, Roumen.  I rather suspected that Py_IS_INFINITY was dodgy this 
way.

On the other hand, this is only a problem when Py_IS_INFINITY is applied 
to a *computed* result;  most of the time Py_IS_INFINITY is being directly 
applied to the incoming argument to some function, so should be safe.