Computes the Pochhammer symbol
(a)_x := \Gamma(a + x)/\Gamma(a),
subject to a and a+x not being negative integers. The Pochhammer symbol is also known as the Apell symbol.
err contains an estimate of the absolute error in the value z.
This function is from the GNU Scientific Library, see http://www.gnu.org/software/gsl/ for documentation.
Package: gsl