Informix external directives ?

Hi all,

Is it possible to use Informix external directives for statements with 
host variables ?

It works fine for statements without host variables.

Now, I need to solve a problem for a statement with a host variable.

e.g.

.... WHERE (a0.t_ncst > ?)

If I add a statement with host variables my directive is not used :-(

I am afraid that I can not benefit from external directives if host 
variables are used.

Can someone confirm this ?

Regards ...

Joerg