foreigner(abdul).
foreigner(marcus).
spouse(edward, sally).
spouse(bill, hillary).
gross_income(hillary, 100000).
gross_income(edward, 20000).
gross_income(bill, 30000).
gross_income(marcus, 49999).
gross_income(mary, 35000).

average_taxpayer(Person) :-
  not(foreigner(Person)),
  not((
    spouse(Person, Spouse),
    gross_income(Spouse, Income1),
    Income1 > 30000 
  )),
  gross_income(Person, Income2),
  Income2 < 50000.