Can anyone explain the advantage of a specification like this over something like UML?
(1) Is constrained in a way designed to enhance understanding of the visual flow, and
(2) Directly includes code (in language Foo) in the boxes, rather than text; that is, its a programming environment, not a diagramming language with text that gets converted to program code somewhere else that is maintained separately.
The first item is also a feature of DRAKON by itself.