[Help-gsl] incomplete gamma function

[Francesco Petrogalli]

Hi,
according to http://people.math.sfu.ca/~cbm/aands/page_260.htm, def.
6.5.3, the Incomplete Gamma function \Gamma(a,x) can be defined for
all values of x, included negative numbers.

Why is GSL computing it only for x>=0?
http://www.gnu.org/software/gsl/manual/html_node/Incomplete-Gamma-Functions.html

I tried to find a way to reduce the negative case to some other
formula but I cannot find it.

Thanks for your help,

Francesco