/*
NOTE: predicates
- a predicate has a name and arguments, number of arguments is called arity
- a predicate with name P and N arguments has a predicate indicator: P/N
- a predicate define a relation between its arguments
- a predicate is defined by a collection of clauses, a clause is either a rule or a fact
*/
/* built-in predicates */
% true: true/0
% definition: true.
?- true. % true.
% false: false/0
% definition: false :- 0 = 1.
?- false. % false.
% conjunction (AND): (',')/2
?- ','(true, true). % true.
?- ','(true, false). % false.
% disjunction (OR): (;)/2
% definition:
% (A ; _) :- A.
% (_ ; B) :- B.
?- ;(true, false). % true
% unification (=): (=)/2
% definition: X = X.
?- =(1, 1). % true.
?- =(1, 2). % false.
% disequality (!=): dif/2
?- dif(1, 2). % true.