Main = link!

link (a,b,x,y,z: list)
[ (n0, x) (n1, y) (n2, z) |
  (e0, n0, n1, a) (e1, n1, n2, b) ]
 => 
[ (n0, x) (n1, y) (n2, z) | 
  (e0, n0, n1, a) (e1, n1, n2, b) (e2, n0, n2, empty) ]
interface = {n0, n1, n2}
where not edge(n0, n2)