Computes the exponential integral E_i(x),
Ei(x) := - PV(\int_{-x}^\infty dt \exp(-t)/t)
where PV denotes the principal value of the integral.
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