Unparse DELETE SQL throws unsupported exception

When i use  SqlDelete.Operator to created  a SqlDelete  , then try to  call toString method unparse to string, it will throw a exception like below:
java.lang.UnsupportedOperationException: class org.apache.calcite.sql.SqlSyntax$7: SPECIALjava.lang.UnsupportedOperationException: class org.apache.calcite.sql.SqlSyntax$7: SPECIAL
at org.apache.calcite.util.Util.needToImplement(Util.java:1080) at org.apache.calcite.sql.SqlSyntax$7.unparse(SqlSyntax.java:128) at org.apache.calcite.sql.SqlOperator.unparse(SqlOperator.java:377) at org.apache.calcite.sql.SqlDialect.unparseCall(SqlDialect.java:453) at org.apache.calcite.sql.SqlCall.unparse(SqlCall.java:103) at org.apache.calcite.sql.SqlNode.toSqlString(SqlNode.java:155)

This exception can be reproduced as below:

```

@Test void testSqlDeleteSqlBasicCallToString() throws Exception {
    final String sql = "delete from employees where id = 1 ";
    final SqlNode sqlNode = parseSql(sql);
    final SqlDelete sqlDelete = (SqlDelete) sqlNode;
    final SqlCall call = SqlDelete.OPERATOR.createCall(sqlDelete.getFunctionQuantifier(),
             sqlDelete.getParserPosition(),sqlDelete.getOperandList());
    System.out.println(call.toString());
}

```