presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/LocalExecutionPlanner/channelGetter
class LocalExecutionPlanner:    private static Function<Symbol, Integer> channelGetter(PhysicalOperation source)
    {
        return input -> {
            checkArgument(source.getLayout().containsKey(input));
            return source.getLayout().get(input);
        };
    }