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