Since Prolog deals with relations and not functions, we can invoke inverse computations in a natural way, unlike functional programming. For instance, in our first example, we can ask
?- edge(3,X).
This asks for all values of X that satisfy the relation edge. If we respond with ; to each answer provided, Prolog will exhaustively list out all possible values that X can take, in the order in which they are defined. The response would look something like
X=4;
X=5;
X=2;
No.
The final No. indicates that there are no further ways of satisfying the goal. Likewise
?- path(3,Y).
will exhaustively list out all values for Y that make this relational assertion true.