This routine computes the Gauss hypergeometric function
\({}_2F_1(a,b,c,x) = F(a,b,c,x)\) for \(|x| < 1\).
If the arguments \((a,b,c,x)\) are too close to a singularity then
the function can return an error when the series approximation
converges too slowly. This occurs in the region of
\(x=1, c - a - b = m\) for integer \(m\).