Combine different Schema classes together

Hi,
I have a form composed of several fieldsets. Since I need to re-use  
those fieldsets over and over again in many forms, I thought to write  
a Schema object for each fieldset, and then combine them in a super- 
validator. Something like this:

class Foo(Schema):
     allow_extra_fields = True
     filter_extra_fields = True
     ... foo validators here ...
     pass

class Bar(Schema):
     allow_extra_fields = True
     filter_extra_fields = True
     ... bar validators here ...
     pass

class Gnus(Schema):
     allow_extra_fields = True
     filter_extra_fields = True
     ... gnus validators here ...
     pass

class SuperSchema(Foo, Bar, Gnus):
     pass

But when I try to validate the super-form using SuperSchema I can get  
only fields defined in Foo, whilst the others are stripped. On the  
other hand defining SuperSchema like this:

class SuperSchema(Foo, Bar, Gnus):
     allow_extra_fields = True
     filter_extra_fields = False
     pass

works correctly, but leave me without protection from malicious fields  
sent by the clients. Is there any other way to make this to work  
correctly without disabling the extra fields filter?

Cheers,
  Andrea

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