This function will store values passed to it (against the property) and set the value of property by dividing the sum of all values by the number of values stored.
E.g. for a property that is set to Average the value of all orders.
Each time an order occurs, the function is passed the value of the order. The function stores the value of each order and sets the value of the property to the sum of all order values divided by the number of orders
Note: This could be used in conjunction with Time Sensitive to produce a property like "Average Order Value for Last 6 Months"