code: private String mergeConfigValue(String key, Set<String> corpora,
      CorpusConfigMap corpusConfigurations)
    {
      Set<String> values = new TreeSet<>();
      for(String corpus : corpora)
      {
        CorpusConfig config = corpusConfigurations.get(corpus);
        if(config != null)
        {
          String v = config.getConfig(key);
          if(v != null)
          {
            values.add(v);
          }
        }
      }
      if(values.size() > 1 || values.isEmpty())
      {
        // fallback to the default values
        CorpusConfig defaultConfig = corpusConfigurations.get(DEFAULT_CONFIG);
        if(defaultConfig != null && defaultConfig.containsKey(key))
        {
          return defaultConfig.getConfig(key);
        }
      }    
  
        // ok, just return the first value as a fallback of the fallback
      if(!values.isEmpty())
      {
        return values.iterator().next();
      }
  
      return null;
    }
docs: If all values of a specific corpus property have the same value, this value
  is returned, otherwise the value of the default configuration is choosen.
  
  @param key The property key.
  @param corpora Specifies the selected corpora.
  @return A value defined in the copurs.properties file or in the
  admin-service.properties