sentences:  A user application is predominately defined, and provided to a PRS system is a set of "knowledge areas".Each knowledge area is a piece of procedural knowledge that specifies how to do something, e.g., how to navigate down a corridor, or how to plan a path (in contrast with robotic architectures where the programmer just provides a model of what the states of the world are and how the agent's primitive actions affect them).Such a program, together with a PRS interpreter, is used to control the agent.