def transition(name)
      t = find_transition name
      if t.nil?
        t = Transition.new name, self
        @transitions << t
      end
      t
    end
def transition(name)
      t = find_transition name
      if t.nil?
        t = Transition.new name, self
        @transitions << t
      end
      t
    end
Create and return new transition for this model.
 +name+ identifies transition in the net.