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By the definition of a message, it must have a meaning. But what is
meaning?
First of all, messages are always addressed to a certain agent.
Also, they are supposed to influence the actions performed by this agent,
otherwise there would no sense in producing and passing the message.
We know of exaclty two ways of changing the actions of an agent.
One way is to change the state of the world as far as the agent is
concerned. The other way is to change the agent's model of the world
(if any). Thus by the type of their semantics messages of a language
break into two categories: commands and statements.
