lhasPizza('bun');
likePizza('bun');
hasPizza('richard');
likePizza('richard');
hasPizza('mary');
likePizza('susan');
//  rule:  head :- body;
//  body: p1, p2, p3 ...;
//  conjunction of predicates.
hasMoney('mary');
happy(X) :- hasPizza(X), likePizza(X);
happy(X) :- hasMoney(X);


//  queries

happy('bun');
>> true
   happy('bun') does not exist directly.
   look for happy(X) :- hasPizza(X), likePizza(X);
   X  binds to 'bun'  (binding)
   look for hasPizza('bun'): true
   look for likePizza('bun'): true

happy(X);
  X = 'bun'
  X = 'richard'