%r3_4.gl

true => e(1,2),e(2,3),e(3,4). % to start with

r(X,Y),r(Y,Z),r(X,Z) => goal.       
g(X,Y),g(Y,Z),g(X,Z) => goal.      

e(X,Y),e(Y,Z) => e(X,Z).   % complete edges

e(X,Y) => r(X,Y);g(X,Y).   % color each edge red or green