code: public String getLowlevelParam(String key) {
          String result;
  
          String intern = getName() + ".sepa.";
          if (key.startsWith(intern)) {
              String realKey = key.substring(intern.length());
              result = getPainParam(realKey);
          } else {
              result = super.getLowlevelParam(key);
          }
  
          return result;
      }
docs: This is needed for verifyConstraints(). Because verifyConstraints() tries
  to read the lowlevel-values for each constraint, the lowlevel-values for
  sepa.xxx would always be empty (because they do not exist in hbci
  messages). So we read the sepa lowlevel-values from the special sepa
  structure instead from the lowlevel params for the message
  
  @param key
  @return the lowlevel param.