@Test
  public void testNegative6() throws ParseException {
    expectedException.expect(RewriteEmptyStreamException.class);
    expectedException.expectMessage("rule whenClauses");
    ASTNode ast = parse(
      "MERGE INTO target USING source ON target.pk = source.pk");
  }
here we reverse the order of WHEN MATCHED/WHEN NOT MATCHED - should we allow it?
@throws ParseException
parser/src/test/org/apache/hadoop/hive/ql/parse/TestMergeStatement.java