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A port is an empty region which touches at most one other region.
A region R satisfies the predicate empty_region if there exists no PJ element inside of R. A port may touch any number of elements which are not regions but at most one other region. Ports serve as docking place for lines.
We distinguish ports with respect to their relationship to other elements. A port may serve as
Any port can be linked or empty (i.e. not linked). However, an empty reference port indicates a semantic error since the element owning this reference port can never be referenced. Therefore, we restrict empty ports to be also argument ports.