presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PushTableWriteThroughUnion.java/PushTableWriteThroughUnion/getInputSymbolMapping
class PushTableWriteThroughUnion:    private static Map<Symbol, Symbol> getInputSymbolMapping(UnionNode node, int source)
    {
        return node.getSymbolMapping()
                .keySet()
                .stream()
                .collect(toImmutableMap(key -> key, key -> node.getSymbolMapping().get(key).get(source)));
    }