link(a, b, c, d, e : list) [ (n1, a) (n2, c) (n3, e) | (e1, n1, n2, b) (e2, n2, n3, d) ] => [ (n1, a) (n2, c) (n3, e) | (e1, n1, n2, b) (e2, n2, n3, d) (e3, n1, n3, empty) ] interface = { n1, n2, n3 } where not edge(1, 3)