code: private GraphCentricQueryBuilder has(String key, TitanPredicate predicate, Object condition) {
          Preconditions.checkNotNull(key);
          Preconditions.checkNotNull(predicate);
          Preconditions.checkArgument(predicate.isValidCondition(condition), "Invalid condition: %s", condition);
          constraints.add(new PredicateCondition<>(key, predicate, condition));
          return this;
      }
docs: /* ---------------------------------------------------------------
  Query Construction
  ---------------------------------------------------------------