Function: eint1
Section: transcendental
C-Name: veceint1
Prototype: GDGp
Help: eint1(x,{n}): exponential integral E1(x). If n is present, computes
 the vector of the first n values of the exponential integral E1(n.x) (x > 0).
Doc: exponential integral $\int_x^\infty \dfrac{e^{-t}}{t}\,dt$ ($x\in\R$)

 If $n$ is present, outputs the $n$-dimensional vector
 $[\kbd{eint1}(x),\dots,\kbd{eint1}(nx)]$ ($x \geq 0$). This is faster than
 repeatedly calling \kbd{eint1($i$ * x)}.
Variant: Also available is \fun{GEN}{eint1}{GEN x, long prec}.
