presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashComputationSet/HashComputationSet
class HashComputationSet:        public HashComputationSet(Optional<HashComputation> hash)
        {
            requireNonNull(hash, "hash is null");
            if (hash.isPresent()) {
                this.hashes = ImmutableSetMultimap.of(hash.get(), hash.get());
            }
            else {
                this.hashes = ImmutableSetMultimap.of();
            }
        }