% exercise on page 273 of Appleby
north_of(X1,X2):-location(X1,Y1,Z1), location(X2,Y2,Z2), Y2 < Y1.

location(new_york, 41,74).
location(chicago, 42,88).
location(tokyo,35,150).
location(oslo,60,11).
location(halto,0,80).
location(cairo,30,30).

a:-north_of(chicago, new_york).
b:-north_of(X, new_york), prin(X),  write(' is north of New York'), nl.

doit:- write('exercise 1a'), nl, a.
doit:- write('exercise 1b'), nl, b.
:-print(' doit, a, b, location, north_of are all loaded...').
:- write('Input the prolog query 'doit?''), nl.