The function "lambert_w" is new to 5.15.0 (thanks due to Raymond and
Stavros).
The solve doesn't know about lambert_w. So far, lambert_w evaluates for
doubles (but not big floats):
(%i5) lambert_w(5.60);
(%o5) 1.392014569377103
(%i6) diff(lambert_w(x),x);
(%o6) %e^(-lambert_w(x))/(lambert_w(x)+1)
(%i9) taylor(lambert_w(x),x,0,4);
(%o9) x-x^2+(3*x^3)/2-(8*x^4)/3+...
Barton