presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/PlanWithProperties/getRequiredHashSymbol
class PlanWithProperties:        public Symbol getRequiredHashSymbol(HashComputation hash)
        {
            Symbol hashSymbol = hashSymbols.get(hash);
            requireNonNull(hashSymbol, () -> "No hash symbol generated for " + hash);
            return hashSymbol;
        }