presto-main/src/main/java/io/prestosql/sql/planner/optimizations/AddExchanges.java/Rewriter/outputToInputTranslator
class Rewriter:        private Function<Symbol, Optional<Symbol>> outputToInputTranslator(UnionNode node, int sourceIndex)
        {
            return symbol -> Optional.of(node.getSymbolMapping().get(symbol).get(sourceIndex));
        }