Now layout files and modules can extend the cite engines or completely
overwrite them, and modify the cite formats.
Any CiteEngine definition in a layout/module will completely overwrite
those by cite engine files.
AddToCiteEngine will extend them (add if they do not exist yet).
Any CiteFormat definition in a layout will be preferred to those in cite
engines. CiteFormat definitions that are not touched by the former are
still active, though (so, as opposed to CiteEngine, a CiteFormat does
not completely overwrite those by the engine files).
Layout format change.
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