OSL - extract alpha

[Mnietek2707]

I'm trying to extract alpha from map (for example multiple png combined with composite node).
Below is my attempt with OSL but this doesn't work.
Maybe someone knows why?

shader extractAlpha
(
string input_tex = "",
output color result = color(0),
output float alpha = 0,
)

{
float f_outputTexAlpha = 0.0;
color C = texture(input_tex, u, v, "alpha", f_outputTexAlpha);;
result = C;
alpha = f_outputTexAlpha;
}