You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: We consider the task-oriented dialogue system that can be modeled as a partially observable Markov decision process (POMDP) (Williams & Young, 2007) deﬁned by tuple (cid:104) S, A, O, T, Z, R, γ (cid:105) where S is the set of environment states s = (cid:104) g, h (cid:105) (underlying state that consists of the user goal g and dialogue history h ), A is the set of actions a (a sequence of tokens which represents dialogue act and system response ), O is the set of observations o (user utterance), T ( s (cid:48) | s, a ) = Pr( s t +1 = s (cid:48) | s t = s, a t = a ) is the transition function, Z ( o | s (cid:48) , a ) = Pr( o t +1 = o | s t +1 = s (cid:48) , a t = a ) is the observation probability, R ( g, h, a ) is the reward function indicating the utility of executing action a in history h and the user goal g , and γ ∈ (0 , 1) is a discount factor. The history at time step t , h t = { o 0 , a 0 , . . . o t − 1 , a t − 1 , o t } , is a sequence of all previous observations and actions. Since the underlying state s (e.g. user goal) is not directly observable, the agent makes decisions based on the entire observation-action history. The policy π ( a t | h t ) is mapping from history h t to a probability distribution over A . The goal is to ﬁnd an optimal policy π ∗ that maximizes the expected cumulative rewards, i.e. π ∗ = arg max π E π [ (cid:80) ∞ t =0 γ t R ( g, h t , a t )] . The action-value function of policy π is deﬁned as Q π ( h, a ) := E π [ (cid:80) ∞ t =0 γ t R ( g, h t , a t ) | h 0 = h, a 0 = a ] , where Q π is a unique solution of the Bellman equation: Q π ( h, a ) = E g [ R ( g, h, a )] + γ E π [ Q π ( h (cid:48) , a (cid:48) )] .

Question: How are actions defined in task-oriented dialogue?
In task-oriented dialogues, the actions are defined as a sequence of tokens which represents dialogue act and system response.