This is a nice overview: CLPFD and CLPZ: Prolog Integer Arithmetic

mother can be used in several ways. If both X and Y variables are uninitialized, then it looks for all mother relationships. If one of them is initialized, it looks for matching relationships. If both are initialized, it returns true if such a relationship exists.

Depends on how you want to define your domain knowledge.

The thing you need to define for sure is the predicate mother/2 (Which has arity 2, or in other words, two arguments). From then on, multiple options are available:

1. Take mother(X, Y) as an “axiom”, and define mother terms for all elements:

mother(abel, eve).
mother(isaac, sarah).

2. Derive mother(X, Y) from female(X) and parent(X, Y) terms.

mother(X, Y) :- 
  parent(X, Y), 
  female(Y).

3. Smash the institutional gender power structures and define only parent/2 terms instead of mother/2 and father/2.