AVGtakes all the values you provide in between a set of brackets, each separated by a comma, and gives you their average.
AVGcan also return the average of values that are produced by another function.
AVG( .... )is the overarching function, of which you want the result. In between its brackets sits another function.
get_properties_from_instances(.......)is the function inside the
AVGbrackets, which goes and fetches the property of the instances you specify.
instances(AccountHolder, personal_income)returns the personal income of all the accountholders.