% Ancestors of Marc and the generation gap.
% Chap 10.4, Advanced Database Systems, C. Zaniolo et al.
%

database( { 
  parent(string, string) 
}).

export delta_ancestor(A, B).

delta_ancestor(0, marc).
delta_ancestor(J+1, Y)	<-  delta_ancestor(J, X), 
			    parent(Y, X),
			    ~all_ancestor(J, Y, _).
all_ancestor(J+1, X, Y) <-  all_ancestor(J, X, Y).
all_ancestor(J, X, X)	<-  delta_ancestor(J, X).