One final thing to note on the subject of reasoning programs is that the language used by an RP will typically be changed with time by the RP. We should expect in general that a reasoning program will find it necessary to define new words or to accept definitions of new words; some of these words will denote new situations, actions, phenomena, or relations--the RP may have to infer the language it uses for solving a problem. Our most important requirement for the initial language is that any necessary extensions to it be capable of being easily added to it.