Can you now define the following predicates:
square_sells( Square, Good, Price ): This holds if Square sells
Good at Price.
square_buys( Square, Good, Price ): This holds if Square buys
Good at price.
at_seller( T, Good, Price ): This is true if trader T is at a
square which sells Good at Price.
at_buyer( T, Good, Price ): This is true if trader T is at a
square which buys Good at Price.
Answers will be found in the file COLLIER, but don't look until you have tried the next section.