Lambert W Function (hp-42s)

[Albert Chan]

Hi, lyuka

I discovered a feature of your code.
Because we turned repeat-until to while-do loop, it is restartable with another guess.

Example, to calculate eW(-log(2)/2), both branch 0 and -1:

2 LN 2 +/- /   → a = -0.3465735902799726547086160607290883
XEQ "eW"       → e^W₀(a) = 0.5
− 0.3 R/S      → e^W_-1(a) = 0.25

For e^W_-1(a), -1/e < a < 0, any positive guess below -a work.

Update: a good guess for e^W_-1(a), -1/e < a < 0, is 2a²
For above case, 2a² already gives 0.24022

Just replace "− new-guess R/S", with this: "− − + × R/S"