/*
area(Chain,Area) :-
Area is the area of the polygon enclosed by the list of points
Chain, where the coordinates of each point are represented by
a pair (X,Y) of integers.
*/
area([Tuple],0).
area([(X1,Y1),(X2,Y2)|XYs],Area) :-
area([(X2,Y2)|XYs],Area1),
Area is (X1*Y2-Y1*X2)/2 + Area1.
% Program 8.8 Computing the area of polygons