Prolog Variable Universality

Rule Mode

For rules, if a variable is defined newly in the rule, then those variables are bound existentially.

sibling(X,Y) :- parent(P, X), parent(P,Y).

Expressed as:

$$\forall X,Y,\text{sibling}(X,Y)\leftarrow \exists P,\text{parent}(P,X)\wedge \text{parent}(P,Y)$$