Agent types

Four basic types:
 * Simple reflex agent
 * Acts according to a rule whose condition matches the current state.
 * Model-based reflex agent
 * Keeps track off the current state of the world, then chooses an action as above.
 * Goal-based agent
 * Keeps track of the world state and a set of goals, then chooses an action that will (eventually) lead to the achievement of its goals.
 * Utility-based agent
 * Uses a utility function that measures its preferences among states of the world, then chooses an action that leads to the best expected utility.