Helper function '_eis(z)' to make 'Ei(z)' and 'li(z)' tractable for the Gruntz algorithm.
Use '_eis(z)' for computing limits of 'li(z)'

For example we can compute that:

In [11]: gruntz(1/li(z), z, oo)
Out[11]: 0
Use '_eis(z)' for computing limits of 'Ei(z)'

For example we can compute that:

In [4]: gruntz(Ei(z), z, oo)
Out[4]: oo

In [5]: gruntz(Ei(z-Ei(z)), z, oo)
Out[5]: 0

This one is obvious (IF we would use the relation between 'Ei' <-> 'li')

In [6]: gruntz(Ei(log(z))/(2*li(z)), z, oo)
Out[6]: 1/2