More facts!

Note how our facts define relations that always go from subject to object, i.e., they describe a directed graph. In a perfect world, friendships would go both ways, i.e. the friendship relation would be symmetric. To model that, we would have to manually assert [:a, :friend, :b] and [:b, :friend, :a], or we can make the engine do the work:

engine.rule "symmetric predicate" do
  forall {
    has :P, :symmetric, true
    has :X, :P, :Y
  make {
    gen :Y, :P, :X

engine << [:friend, :symmetric, true]

If you do this on the engine with the self-printer rule installed, you'll instantly see the new reciprocated friendships pop up.

gen is an action that creates new facts based on the provided template and the variables bound in the token, and it's one of the most important elements in a rule. Generated actions can trigger other rules, which is a very powerful mechanism.