Undoes and the redoes (re-applies) an expression identified by a tag.

  pdef = Ruote.process_definition do
    sequence do
      alpha :tag => 'kilroy'
      _redo :ref => 'kilroy', :unless => '${f:ok} == true'
    end
  end

will redo at tag ‘kilroy’ if the field ‘ok’ is not set to true.

(redo is escaped as _redo not to conflict with the "redo" Ruby keyword)

Maybe this case is better served by a cursor/rewind combination

  pdef = Ruote.process_definition do
    cursor do
      alpha :tag => 'kilroy'
      rewind :unless => '${f:ok} == true'
    end
  end