module path.

kind node  type.
type adj   node -> node -> o.
type path  node -> node -> o.

type a     node.
type b     node.
type c     node.
type d     node.
type e     node.
type f     node.

adj a b.
adj a c.
adj b c.
adj c d.
adj e d.

path X Y :- adj X Y.
path X Z :- adj X Y, path Y Z.

% You can also write
% path X Z :- sigma y\(adj X Y, path Y Z).
% but don't write
% path X Z :- path X Y, path Y Z.