% negation.dl -- Stratified negation and comparison operators
%
% Demonstrates negation-as-failure (not) and built-in comparison
% predicates in Datalog rule bodies.

% --- Facts ---

node(a).
node(b).
node(c).
node(d).
node(e).

infected(b).
infected(d).

employee(alice, 55000).
employee(bob, 72000).
employee(carol, 48000).
employee(dave, 91000).
employee(eve, 63000).

% --- Rules with negation ---

% Safe nodes: not infected
safe(X) :- node(X), not infected(X).

% --- Rules with comparison ---

% High earners: salary > 60000
high_earner(Name) :- employee(Name, Salary), Salary > 60000.

% Low earners: salary <= 50000
low_earner(Name) :- employee(Name, Salary), Salary <= 50000.

% Earns more than another
earns_more(X, Y) :- employee(X, Sx), employee(Y, Sy), Sx > Sy.

% Same salary
same_salary(X, Y) :- employee(X, S), employee(Y, S), X \= Y.

% --- Queries ---

?- safe(X).
?- high_earner(X).
?- earns_more(dave, X).