undo

Undoes (cancels) another expression referred by its tag.

              pdef = Ruote.process_definition do
                concurrence do
                  alpha :tag => 'kilroy'
                  undo :ref => 'kilroy'
                end
              end

This example is rather tiny, but it shows a process branch (undo) cancelling
another (alpha).

cancel

This expression is aliased to ‘cancel’

              cancel :ref => 'invoicing_stage'

a bit shorter

It’s OK to shorten

              cancel :ref => 'invoicing_stage'

to

              cancel 'invoicing_stage'