presto-main/src/main/java/io/prestosql/sql/planner/planprinter/PlanPrinter.java/PlanPrinter/formatHash
class PlanPrinter:    private static String formatHash(Optional<Symbol>... hashes)
    {
        List<Symbol> symbols = stream(hashes)
                .filter(Optional::isPresent)
                .map(Optional::get)
                .collect(toList());

        if (symbols.isEmpty()) {
            return "";
        }

        return "[" + Joiner.on(", ").join(symbols) + "]";
    }