protected MergeStatement.DeleteClause handleDelete(
      ASTNode whenMatchedDeleteClause, String updateExtraPredicate) {
    assert whenMatchedDeleteClause.getType() == HiveParser.TOK_MATCHED;
    String extraPredicate = getWhenClausePredicate(whenMatchedDeleteClause);
    return new MergeStatement.DeleteClause(extraPredicate, updateExtraPredicate);
  }
@param updateExtraPredicate - see notes at caller
ql/src/java/org/apache/hadoop/hive/ql/parse/MergeSemanticAnalyzer.java