presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPropertyDerivations.java/StreamPropertyDerivations/computeIdentityTranslations
class StreamPropertyDerivations:        private static Map<Symbol, Symbol> computeIdentityTranslations(Map<Symbol, Expression> assignments)
        {
            Map<Symbol, Symbol> inputToOutput = new HashMap<>();
            for (Map.Entry<Symbol, Expression> assignment : assignments.entrySet()) {
                if (assignment.getValue() instanceof SymbolReference) {
                    inputToOutput.put(Symbol.from(assignment.getValue()), assignment.getKey());
                }
            }
            return inputToOutput;
        }