code: protected void assertLegalRelativeAddition(String relativePropertySourceName, PropertySource<?> propertySource) {
          String newPropertySourceName = propertySource.getName();
          if (relativePropertySourceName.equals(newPropertySourceName)) {
              throw new IllegalArgumentException(
                  "PropertySource named '" + newPropertySourceName + "' cannot be added relative to itself");
          }
      }
docs: Ensure that the given property source is not being added relative to itself.