code: @Override
    public Properties getConfiguration(Environment environment) {
      Properties properties = new Properties();
  
      for (ConfigurationSource source : sources) {
        Properties sourceProperties = source.getConfiguration(environment);
        properties.putAll(sourceProperties);
      }
  
      return properties;
    }
docs: Get configuration set for a given {@code environment} from this source in a form of {@link Properties}. The configuration
  set is a result of a merge of provided {@link ConfigurationSource} configurations. In case of key collision
  last-match wins merge strategy is used.
  
  @param environment environment to use
  @return configuration set for {@code environment}
  @throws MissingEnvironmentException when requested environment couldn't be found
  @throws IllegalStateException       when unable to fetch configuration from one of the underlying sources