prolog-recursion-2.pro

% Multiple rules with the same signature:
ancestor(X, Y) :- father(X, Y).                 % base case first
ancestor(X, Y) :- father(X, Z), ancestor(Z, Y). % we recursively call ancestor

% Alternatively, we can write it this way using a logical OR:
ancestor(X, Y) :- father(X, Y);                 % we use a semicolon to distinguish between clauses
                  father(X, Z), ancestor(Z, Y).