Goals, Actions, and States
   - A relatively simple but useful concept is the goal.
 
   - In a maze, the goal may be to get to the other side.
 
   - When packing your shopping, the goal may be to have your
       groceries in a reasonable number of bags.
 
   - As ever, the domain can be represented by States.
 
   - In the maze, the state is the location; and in shopping it may
     be the location of your groceries, or it may be to have it in
     the minimum number of bags.
 
   - Actions change the state.
 
   - Moving, changes the location in the maze.
 
   - Putting your milk in a bag will change the State.
 
   - Note that all actions may not be available.
 
   - You can use preconditions.  So, you can't put milk in a bag
     that is too full, or you can't put milk on top of something that
     isn't already in the bag.