I create an empty class with an empty method.
I create a sequence diagram that defines the empty method's contents.
I choose the activation on that empty method and select "Generate
implementation".
The forward-engineering option to 'never overwrite source code' for sequence
diagrams (both project and diagram level) is NOT selected.
However, Together will not generate the method content. It only reports
"Cannot generate implementation for the selected element.
Nothing to do."