Package: pari
Version: 2.6.1

Could mplambertW be extended for -1/e <= y < 0? I have an implementation, but it's pretty easy to code (series expansion + Householder iteration). There is a branch cut choice to be made, perhaps with an optional parameter or perhaps just forcing branch 0.

I just came across an application for this and was surprised to see this part of the domain not implemented.