presto-main/src/main/java/io/prestosql/sql/planner/SortExpressionExtractor.java/SortExpressionExtractor/merge
class SortExpressionExtractor:    private static SortExpressionContext merge(SortExpressionContext left, SortExpressionContext right)
    {
        checkArgument(left.getSortExpression().equals(right.getSortExpression()));
        ImmutableList.Builder<Expression> searchExpressions = ImmutableList.builder();
        searchExpressions.addAll(left.getSearchExpressions());
        searchExpressions.addAll(right.getSearchExpressions());
        return new SortExpressionContext(left.getSortExpression(), searchExpressions.build());
    }