The ‘add_branches’/‘add_branch’ expression can be used to add branches
to a concurrent-iterator while it is running.

              concurrent_iterator :on => 'a, b, c' do
                sequence do
                  participant :ref => 'worker_${v:i}'
                  add_branches 'd, e', :if => '${v:/not_sufficient}'

In this example, if the process level variable ‘not_sufficient’ is set to
true, workers d and e will be added to the iterated elements.

‘add_branches’ understand comma-separated list of values or direcltly
array of values, like the concurrent_iterator does. The :sep or :separator
attribute can be used for custom separators :

              add_branches 'd|e|f', :sep => '|'


By default, add_branches looks up the first parent expression that is
concurrent_iterator. This is all well, but what when you have nested
concurrent_iterator and want to hit the enclosing one from inside the
enclosed one ? Or when you want to add branches from somewhere else
in the process instance, outside of the concurrent_iterator ?

              concurrence do
                concurrent_iterator :on => 'a, b, c', :tag => 'main' do
                  subprocess :ref => 'perform_work'
                sequence do
                  subprocess :ref => 'supervise_work'
                  add_branches 'd, e', :ref => 'main', :if => '${f:more_cowbell}'
                  rewind :if => '${f:more_cowbell}'

The add_branches expression refers to the ‘main’ concurrent_iterator via
the :ref => ‘main’ attribute.

missing concurrent_iterator

If :ref points to nothing or add_branch has no :ref and is not placed
inside of a concurrent_iterator, the expression will silently have no