.type Node <: symbol

.decl X(node1:Node, node2:Node)
.output X()
.decl Y(node1:Node, node2:Node)
.output Y()

Y(a,b) :- X(a,b).
Y(a,c) :- Y(a,b), Y(b,c).

X("a","b").
X("b","d").
X("a","c").
X("c","e").
