Function: incgam
Section: transcendental
C-Name: incgam0
Prototype: GGDGp
Help: incgam(s,x,{y}): incomplete gamma function. y is optional and is the
 precomputed value of gamma(s).
Doc: incomplete gamma function. The argument $x$
 and $s$ are complex numbers ($x$ must be a positive real number if $s = 0$).
 The result returned is $\int_x^\infty e^{-t}t^{s-1}\,dt$. When $y$ is given,
 assume (of course without checking!) that $y=\Gamma(s)$. For small $x$, this
 will speed up the computation.
Variant: Also available is \fun{GEN}{incgam}{GEN s, GEN x, long prec}.
