% XY Example 1. Reachable nodes.
% "Negation and Aggregates in Recursive Rules: the LDL++ Approach", DOOD 93.

database ( {
        g(X:string, Y:string)
}).

export delta(I,X).
export all(I,X).
export delta($I,X).



delta(0, Y) <- g(a, Y).
delta(I+1, Y) <- delta(I, X), g(X, Y), ~all(I, Y).

all(I+1, X) <- all(I, X), delta(I+1, _).
all(I, Y) <- delta(I, Y).