presto-main/src/main/java/io/prestosql/sql/planner/optimizations/PlanNodeDecorrelator.java/PlanNodeDecorrelator/getCorrelatedSymbolMapper
class PlanNodeDecorrelator:        SymbolMapper getCorrelatedSymbolMapper()
        {
            return new SymbolMapper(correlatedSymbolsMapping.asMap().entrySet().stream()
                    .collect(toImmutableMap(Map.Entry::getKey, symbols -> Iterables.getLast(symbols.getValue()))));
        }