Chained validators and error messages

Hi,
I'm pretty new to Formencode, so please forgive my inexperience.

I'm trying to make htmlfill to put the error message raised by a  
chained validator near a specific form field.

To get this I've written a custom validator where the messages dict is  
defined like this:

messages = {'my_error_label': u"Error message goes here" }

then, if the conditions are not met I raise the Invalid exception like  
this:

raise Invalid(self.message('my_error_label', state),
values, state,
error_dict={
'my_error_label': Invalid(self.message('field_name', state), values,  
state)
})

Despite the fact this gives me the expected result, I'm not sure I'm  
raising that exception in the correct way. I mean, I got the  
impression that the Invalid instance could be called in easier way,  
but I really can't see how. Any suggestion?

TIA,
  Andrea

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