code: private static IsolationLevel selectIsolationLevel(DatabaseMetaData md,
                                                         IsolationLevel desiredLevel)
          throws SQLException, RepositoryException
      {
          while (!md.supportsTransactionIsolationLevel(mapIsolationLevelToJdbc(desiredLevel))) {
              switch (desiredLevel) {
              case READ_UNCOMMITTED:
                  desiredLevel = IsolationLevel.READ_COMMITTED;
                  break;
              case READ_COMMITTED:
                  desiredLevel = IsolationLevel.REPEATABLE_READ;
                  break;
              case REPEATABLE_READ:
                  desiredLevel = IsolationLevel.SERIALIZABLE;
                  break;
              case SNAPSHOT:
                  desiredLevel = IsolationLevel.SERIALIZABLE;
                  break;
              case SERIALIZABLE: default:
                  return null;
              }
          }
          return desiredLevel;
      }
docs: Returns the highest supported level for the given desired level.
  
  @return null if not supported