randomIndex	A seed for random number selection that can be used to ensure repeatability
price[1-6]	The prices of the six items for sale
width* (in comments)	The diameters of the 8 coins - width20 is the width of a 20p piece etc
ix[1-8]	The indices of the eight coins in the purse in the list of all coins [1,2,5,10,20,50,100,200]
coinsselected	An array of 8 boolean values to record which of the coins in the purse have been selected
itemsselected	An array of 6 integer values to record how many instances of each of the items have been selected
imagelocation	The web address at which the images being retrieved are stored
scaleWidth	The scaling factor that is used in specifying the screen layout

Observables defined by dependency

item[1-6]	The six items that are for sale, each recorded as a pair comprising a name and price
forsale	The complete list of items that are for sale, recorded as [item1, item2, .... , item6]
items	The list comprising the six items that are for sale, each recorded as a pair comprising a name and price
basket	The list of names of items that have been selected for purchase
bill	The total cost of the items that have been selected for purchase
coinlist	The list of coins, in which each denomination is recorded as an integer number of pence
purse	The coins that the customer has available to spend
spendingmoney	The total amount of money that the customer has to spend
haveenoughmoney	Whether or not the cost of the currently selected items exceeds the total amount of money the customer has to spend
shortfall	How far the cost of the currently selected items exceeds the total amount of money the customer has to spend or zero if the customer has enough money
tendered	The amount of money the customer is offering for the items selected
change	The amount of change the customer would be entitled to, or zero if the customer hasn't offered enough money
coin[1-8]pic	The images used to depict the eight denominations of coin that can appear on the display
item[1-6]pic	The images used to depict each of the six items selected (where appropriate) on the display
item[1-6]pic0	The images used to depict each of the six items for sale on the display
coin[1-8]text	The text used to display the denominations of the coins belonging to the customer
item[1-6]numtext	The text used to display the number of instances of each item selected by the customer
item[1-6]text	The text used to display the price of each item for sale
PurseContentText	The text used to designate the content of the purse on the display
CoinsInHandText	The text used to designate the coins tendered on the display
BasketContentText	The text used to designate the content of the basket on the display
ItemsForSaleText	The text used to designate the items for sale on the display
picture	The built-in observable used to specify the list of drawable components on the display
optpic[1-6]	The optional drawable components used to display the items that have been selected by the customer

The following user-defined functions are used to express dependency relationships:

listitems	Making a list of the selected items
costitems	Calculating the cost of the selected items
moneyinhand	Calculating the amount of money tendered
coindisplay	Determining which image to display given an index of a coin in the list of coins
listcoins	Making a list of the tendered coins
itemdisplay	Determining which image to display given an index of an item in the list of items for sale
denomdisplay	Determining what text to display given an index of a coin in the list of coins
displaycurrency	Determining how to display a given quantity of money that is expressed in pounds
pricedisplay	Determining what price to display given an index of an item in the list of items for sale
mkstr	Displaying a non-zero integer as a string whilst displaying zero by the empty string

The following agents are used to perform actions:

coin[1-8]picMove	The mouse click actions for selecting/deselecting coins
item[1-6]picMove	The mouse click actions for returning selected items
item[1-6]pic0Move	The mouse click actions for selecting items