When you want to mix several types of UML diagrams, you can add the word "allow_mixing" right after @start_uml
But then declaring interfaces does not work anymore.
() "Does not work"
I'd gues this is becasue the word "interface" as a double meaning: In class diagrams and in component diagrams.
Would be nice to be able to distinguish. For example "interface" is a class diagram interface. and "()" is a round circle interface from the component diagram.