presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashComputationSet/withHashComputation
class HashComputationSet:        public HashComputationSet withHashComputation(Optional<HashComputation> hashComputation)
        {
            if (!hashComputation.isPresent() || hashes.containsKey(hashComputation.get())) {
                return this;
            }
            return new HashComputationSet(ImmutableSetMultimap.<HashComputation, HashComputation>builder()
                    .putAll(hashes)
                    .put(hashComputation.get(), hashComputation.get())
                    .build());
        }