MACHINE           CoffeeClub1

VARIABLES         piggybank

INVARIANT         piggybank : NAT

INITIALISATION    piggybank := 0

OPERATIONS

  FeedBank(amount) =
    PRE amount : NAT 
    THEN piggybank := piggybank + amount
    END;

  RobBank(amount) =
    PRE amount : NAT & amount <= piggybank
    THEN piggybank := piggybank - amount
    END;

  money <-- CashLeft =
    BEGIN money := piggybank END
    END