Creating new operators & function under proto

Hi,

I tried follwoing the tutorial to create new functions to use in Proto 
but it failed.
Let's say I want to have a new unary function called worker that create 
an expression
that ought to be matched by the following grammar :

struct worker_grammar : or_< terminal< user_func<_> >, function< 
worker_tag, terminal< user_func<_> > > {};
I keep getting a 'no type named proto_expr_base in worker_tag' error.

Related question, how can I make a new function which tag is a tempalte 
class whose tempalte parameter is used to change how the associated 
transform is performed, I tried :

struct worker_grammar : or_< terminal< user_func<_> >, function< 
worker_tag<_>, terminal< user_func<_> > > {};
but it failed too.

Thanks in advance

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Joel FALCOU
Research Engineer @ Institut d'Electronique Fondamentale
Université PARIS SUD XI
France