+ # Declare a base compound (usually, a base class).
+ #
+ # Parameters:
+ #
+ # * `id`: `model_id` of the base compound. May be empty.
+ # * `full_name`: qualified name of the base compound. May be empty.
+ # * `prot`: visibility (proctection) of the relationship.
+ # * `virt`: level of virtuality of the relationship.