docs: public void addBefore(String relativePropertySourceName, PropertySource<?> propertySource) {
          if (logger.isDebugEnabled()) {
              logger.debug("Adding PropertySource '" + propertySource.getName()
                           + "' with search precedence immediately higher than '" + relativePropertySourceName + "'");
          }
          assertLegalRelativeAddition(relativePropertySourceName, propertySource);
          removeIfPresent(propertySource);
          int index = assertPresentAndGetIndex(relativePropertySourceName);
          addAtIndex(index, propertySource);
      }