(28/48/50) Lambert W Function

[Gil]

I tried and changed the code fo my function FNL relative to ln(1+x)-x

FNL=
«
WHILE x .4 >=
REPEAT 'x/(sqrt(1+x)+1)' ->NUM 'x' STO x FNL 2 * x SQ - 'fnl' STO
END 'x/(x+2)' ->NUM 'X2' STO X2 DUP * 'X4' STO 'X4*(5005-X4*(5082-X4*969))/(15015-X4*(24255-X4*(11025-X4*1225)))' ->NUM 'X4' STO 'X2*(X4+X4-x)' ->NUM 'fnl' STO
»

But, for initial x=4, I get a final fnl = -4.77474540488E-2 that is incorrect, of course.