foo :-
    inc(X, Y),         % Implicit RELATION.
    X = zero,          % MATCH X against the atom zero, impliclity match Y.
    something(X, Y).   % same as something(zero, succ(zero)).

  bar :-
    inc(X, Y),         % Same implicit relation.
    Y = succ(zero),    % Implicitly match X against zero.
    something(X, Y).   % same as something(zero, succ(zero)).