In SUD, the subj main relation is used for all subjects (either nominal which are nsubj in UD or clausal which are csubj in UD).
subj is never used with a subrelation.
Most of the times, subj is used without deep feature.
Possible deep feature extensions are:
Other extensions appear during conversion from UD => SUD, UD subrelation are kept as “deep” SUD relation for information preserving.
For instance UD:nsubj:cop => SUD:subj@cop.
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