The simpcheck converts taylor polynomial inputs to non-taylor polynomials.
OK, but not ideal (current code)
(%i2) lambert_w(taylor(x,x,0,5));
(%o2) lambert_w(x+...)
Your code:
(%i5) lambert_w(taylor(x,x,0,5));
(%o5) lambert_w(x)
See how the trig functions work (the taylorize function):
(%i6) cos(taylor(x,x,0,5));
(%o6) 1-x^2/2+x^4/24+...
The taylorize code might need to be revisited -- it might
be overly complicated; I haven't looked at it recently.
Barton