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'