ref

Sometimes you don’t know at ‘design time’, if you want to trigger a
participant or subprocess.

              Ruote.process_definition do
                sequence do
                  participant 'alice'
                  ref '${solver}'
                  participant 'charlie'
                end
              end
            

In this process, solver’s name could be a participant name or a subprocess
name.

Subprocesses have the priority over participants.

Note: this expression is used by the worker when substituting unknown
expression names with participant or subprocess refs.