lambertw, all branches

[Gil]

Any real tiny value will always do the job:
(i×b+ real_tiny) × EXP ((i×b+ real_tiny)
=(i×b+ real_tiny) × EXP (i×b) × EXP (real_tiny)
=(i×b+ real_tiny) × EXP (i×b) × 1
=(i×b+ real_tiny) × EXP (i×b)
=(i×b) × EXP (i×b)
=B×EXP(B)

W(B×EXP(B) =B=(i×b)
==(i×b+ real_tiny)