Problem extending SilvaNews

We use the SilvaNews product and want to create a new product
extending SilvaNews with new content types.

Our new content types have some new properties (short_teaser,
authors_info, ...), extending the ones of SilvaNews content types
(location, subjects, target_audiences,...).

In order to edit our new content types (myobject/edit/tab_edit) using
kupu, we have to extend the list of editable properties we can see
into the right column of the kupu-editor view (where location,
subjects and target_audiences in SilvaNews already appear).

Problem: this part of code is using a link to infrae in order to build
the 'metas' variable into the kupu_content.py script:

scheme = 'http://infrae.com/namespaces/metadata/silva-news'
meta_template = (
   '<meta scheme="http://infrae.com/namespaces/metadata/silva-news" '
   'name="%s" content="%s" />')

metas = []

-> how can I include the new properties (short_teaser, authors_info,
...) of my new content types here in order to edit them using the
kupu-editor view?

Cheers
Pierre

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